I have just dumped code into a Chrome console and saved a cert while in a pinch. It’s not best practices of course, but when you need something fast for one-time use, it’s nice to have something immediately available.
You could make your own webpage that works in the browser (no backend) and make a cert. I haven’t published anything publicly because you really shouldn’t dump private keys in unknown websites, but nothing is stopping you from making your own.
Tunic
Sea of Stars