sendToKindleButton.js

// this could be injected to every page by Safe JS Injector

function sendToKindle(){
//    copy here instapaper send to kindle bookmarklet code
}

const button = document.createElement('div');
button.onclick = sendToKindle;
button.style.display= 'block';
button.style.position= 'absolute';
button.style.background= 'red';
button.style.width= '30px';
button.style.height= '20px';
button.style.top= '0';
button.style.left= '0';
button.style.zIndex= '10000';

window.addEventListener('load', () => {
    document.body.appendChild(button);
})