thingiversedl.user.js

// ==UserScript==
// @name         thingiverse dl
// @namespace    http://tnraro.com/
// @version      0.1
// @author       tnRaro
// @match        https://www.thingiverse.com/thing:*/files
// @grant        none
// ==/UserScript==

(function() {
  'use strict';

  const render = () => {
    const $header = document.querySelector('[class*="ThingFilesListHeader__header"]');
    if ($header == null) {
      setTimeout(render, 100);
      return;
    }
    $header.style.alignItems = "center";
    const $download = document.createElement("a");
    $download.href = `${location.href.replace(/files$/, "")}zip`;
    $download.textContent = "Download All Files"
    $header.append($download);
  }
  window.addEventListener("load", render);
})();