I have been through several iterations of scarfing jigs for my chop saw, each one simplifer than the last. It doesn't get any simpler than this one:-

I stick a thick piece of scrap on the chopsaw table using double-sided tape, and chop it off at the scarf angle. The scrap must be wide enough to reach out well beyond the saw slot. Then I move the blade back to 90 deg. I just hold the piece to be scarfed against the guide and chop it. The important thing is to put a spring clamp on the workpiece at the end of the guide to stop the blade pulling the piece in.

It may sound crude, but with a bit of care, this works as well or better than anything else I've tried.