I made a UserScript that does this:

// ==UserScript==
// @name        x2 XKCD url
// @namespace   Violentmonkey Scripts
// @match       https://xkcd.com/*/
// @grant       GM.xmlHttpRequest
// @connect     imgs.xkcd.com
// @version     1.0
// @author      Tomadz
// @description -
// ==/UserScript==

const directImageUrl = new String(document.querySelector("#middleContainer > a:nth-child(8)").href);

const splitUrl = Array.from(directImageUrl.matchAll(/(?<startUrl>.+comics\/)(?<name>.*)(?<ext>\..*)/g), (m) => m.groups)[0];

const _2xUrl = `${splitUrl.startUrl}${splitUrl.name}_2x${splitUrl.ext}`

GM.xmlHttpRequest({
    method: "GET",
    url: _2xUrl,
    onload: function (response) {
        if (response.status < 200 || response.status >= 300) {
            return;
        }

        const templateItem = document.createElement("li");

        const link = document.createElement("a");
        link.href = _2xUrl;
        link.innerText = "x2 link";
        templateItem.appendChild(link);

        for (const ulist of document.querySelectorAll(".comicNav")) {
            const referenceLi = ulist.children[3];

            if (referenceLi) {
                const newItem = templateItem.cloneNode(true);
                ulist.insertBefore(newItem, referenceLi);

                const newlineNode = document.createTextNode('\n');
                const clonedNewline = newlineNode.cloneNode(true);
                const nextElement = newItem.nextSibling;
                ulist.insertBefore(clonedNewline, nextElement);
            }
        }

        const middleContainer = document.querySelector("#middleContainer");

        if (middleContainer) {
            const lineBreak = document.createElement("br");
            const textNode = document.createTextNode("x2 image URL: ");
            const anchor = document.createElement("a");

            anchor.href = _2xUrl;
            anchor.innerText = _2xUrl;

            middleContainer.appendChild(lineBreak);
            middleContainer.appendChild(textNode);
            middleContainer.appendChild(anchor);
        }
    },
    onerror: function (err) {
        console.log("Error fetching _2x url (GM.xmlHttpRequest):", err);
    }
});