How to load an external script in an iframe that will reference the iframe's document not the parent document

Question

I am using an iframe in order to isolate its content's CSS. Which is working great. But, I need the iframe to load an external script, specifically the script I am loading is: https://web.squarecdn.com/v1/square.js

You can see that this external script calls document often. I need it to reference the document of my iframe not the document of the parent HTML.

I have tried this and it does not accomplish this:

const insertScriptToIframe = (doc, target, src, callback) => {
    var s = doc.createElement("script");
    s.type = "text/javascript";
    if(callback) {
        if (s.readyState){  //IE
            s.onreadystatechange = function(){
                if (s.readyState == "loaded" ||
                    s.readyState == "complete"){
                    s.onreadystatechange = null;
                    callback();
                }
            };
        } else {  //Others
            s.onload = function(){
                callback();
            };
        }
    }
    s.src = src;
    target.appendChild(s);        
}


// add web payments script
const createWebPaymentsNode = () => {
    const iFrame = document.createElement('iframe');
    const context = iFrame.contentDocument;
    const frameHead = context.getElementsByTagName('head').item(0);
    insertScriptToIframe(context, frameHead, 'https://web.squarecdn.com/v1/square.js');
}

It adds the external script to the iframe. But the external script is referencing the parent document. I know it is doing this because the external script, itself, loads other scripts and iframes that are being placed in the parent HTML, instead of the iframe's.

I need to be able to do this in vanilla js. Thank you