A read only s3 permissions policy. think s3://outbounddrops/client-name, where you give

read-only-s3-permission-subkey-policy.json

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": [
        "s3:ListAllMyBuckets",
        "s3:GetBucketLocation"
      ],
      "Effect": "Allow",
      "Resource": [
        "arn:aws:s3:::*"
      ]
    },
    {
      "Effect": "Allow",
      "Action": "s3:ListBucket",
      "Resource": "arn:aws:s3:::{bucket}",
      "Condition": {
        "StringLike": {
          "s3:prefix": [
            "",
            "{key}",
            "{key}/*"
          ]
        }
      }
    },
    {
      "Action": [
        "s3:GetObject"
      ],
      "Effect": "Allow",
      "Resource": [
        "arn:aws:s3:::{bucket}/{key}",
        "arn:aws:s3:::{bucket}/{key}/*"
      ]
    }
  ]
}