thingiverse.user.js

// ==UserScript==
// @name         Thingiverse Zip Button
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  try to take over the world!
// @author       You
// @match        https://www.thingiverse.com/thing:*
// @icon         https://www.google.com/s2/favicons?sz=64&domain=thingiverse.com
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    function addDownloadZipLink() {
        const downloadAllLink = document.querySelector('[class^="SidebarMenu__download"]');
        const downloadContainer = downloadAllLink.parentElement;

        if(downloadContainer.dataset.hasZipLink) return;

        const thingId = document.location.pathname.match(/thing:(\d*)/)[1];

        const zipImage = document.createElement('img');
        zipImage.src = 'https://upload.wikimedia.org/wikipedia/commons/b/be/Octicons-file-zip.svg';
        zipImage.height = 24;
        zipImage.style.float = 'left';
        zipImage.style.filter = 'invert(1)';
        zipImage.style.padding = '3px 8px';

        const button = document.createElement('div');
        button.className = 'button button-primary';
        button.style.textAlign='left';
        button.style.fontWeight = '600';
        button.appendChild(zipImage)
        button.append("Download as Zip File");

        const link = document.createElement('a');
        link.className = downloadAllLink.className;
        link.href = `/thing:${thingId}/zip`

        link.appendChild(button);

        downloadContainer.prepend(link);

        downloadAllLink.firstChild.style.opacity = '30%';
        downloadContainer.dataset.hasZipLink = true;
    }

    const observer = new MutationObserver(addDownloadZipLink)
    observer.observe(document.body, {childList:true, subtree:true});
})();