Importing this in GTM will create the lookup table variable. Don't forget to fill in the lookup table before using this!

lookup_table_container.json

{
    "exportFormatVersion": 2,
    "exportTime": "2019-04-15 07:13:17",
    "containerVersion": {
        "path": "",
        "accountId": "4701686890",
        "containerId": "11602292",
        "containerVersionId": "0",
        "container": {
            "path": "",
            "accountId": "",
            "containerId": "",
            "name": "",
            "publicId": "",
            "usageContext": [
                "WEB"
            ],
            "fingerprint": "",
            "tagManagerUrl": ""
        },
        "tag": [
            {
                "accountId": "4701686890",
                "containerId": "11602292",
                "tagId": "1",
                "name": "Pageview Tag",
                "type": "ua",
                "parameter": [
                    {
                        "type": "BOOLEAN",
                        "key": "overrideGaSettings",
                        "value": "false"
                    },
                    {
                        "type": "TEMPLATE",
                        "key": "trackType",
                        "value": "TRACK_PAGEVIEW"
                    },
                    {
                        "type": "TEMPLATE",
                        "key": "gaSettings",
                        "value": "{{UA ID}}"
                    }
                ],
                "fingerprint": "1555312382153",
                "firingTriggerId": [
                    "2147479553"
                ],
                "tagFiringOption": "ONCE_PER_EVENT"
            }
        ],
        "variable": [
            {
                "accountId": "4701686890",
                "containerId": "11602292",
                "variableId": "2",
                "name": "UA ID",
                "type": "gas",
                "parameter": [
                    {
                        "type": "TEMPLATE",
                        "key": "cookieDomain",
                        "value": "auto"
                    },
                    {
                        "type": "BOOLEAN",
                        "key": "doubleClick",
                        "value": "false"
                    },
                    {
                        "type": "BOOLEAN",
                        "key": "setTrackerName",
                        "value": "false"
                    },
                    {
                        "type": "BOOLEAN",
                        "key": "useDebugVersion",
                        "value": "false"
                    },
                    {
                        "type": "BOOLEAN",
                        "key": "useHashAutoLink",
                        "value": "false"
                    },
                    {
                        "type": "BOOLEAN",
                        "key": "decorateFormsAutoLink",
                        "value": "false"
                    },
                    {
                        "type": "BOOLEAN",
                        "key": "enableLinkId",
                        "value": "false"
                    },
                    {
                        "type": "BOOLEAN",
                        "key": "enableEcommerce",
                        "value": "false"
                    },
                    {
                        "type": "TEMPLATE",
                        "key": "trackingId",
                        "value": "{{UA lookup table}}"
                    }
                ],
                "fingerprint": "1555312359897"
            },
            {
                "accountId": "4701686890",
                "containerId": "11602292",
                "variableId": "1",
                "name": "UA lookup table",
                "type": "smm",
                "parameter": [
                    {
                        "type": "BOOLEAN",
                        "key": "setDefaultValue",
                        "value": "false"
                    },
                    {
                        "type": "TEMPLATE",
                        "key": "input",
                        "value": "{{Page Hostname}}"
                    },
                    {
                        "type": "LIST",
                        "key": "map",
                        "list": [
                            {
                                "type": "MAP",
                                "map": [
                                    {
                                        "type": "TEMPLATE",
                                        "key": "key"
                                    },
                                    {
                                        "type": "TEMPLATE",
                                        "key": "value"
                                    }
                                ]
                            }
                        ]
                    }
                ],
                "fingerprint": "1555312310855",
                "formatValue": {}
            }
        ],
        "builtInVariable": [
            {
                "accountId": "4701686890",
                "containerId": "11602292",
                "type": "PAGE_URL",
                "name": "Page URL"
            },
            {
                "accountId": "4701686890",
                "containerId": "11602292",
                "type": "PAGE_HOSTNAME",
                "name": "Page Hostname"
            },
            {
                "accountId": "4701686890",
                "containerId": "11602292",
                "type": "PAGE_PATH",
                "name": "Page Path"
            },
            {
                "accountId": "4701686890",
                "containerId": "11602292",
                "type": "REFERRER",
                "name": "Referrer"
            },
            {
                "accountId": "4701686890",
                "containerId": "11602292",
                "type": "EVENT",
                "name": "Event"
            }
        ],
        "fingerprint": "0",
        "tagManagerUrl": ""
    }
}