I have a tree skirt pattern that requires a 9 or 10 degree wedge ruler.

Already have way too many specialty rulers, most never used,
so hate buying another. Has anyone come up with a way of
cutting without a wedge ruler?

You could try the old pencil with a string trick. Pin down one edge of the string at the folded cornor and make the string the length you need and tie a pencil to it. :-D

You could make a paper template and tape it to the ruler, then line up the edge of the paper with the cut edge of the fabric. I have done that with trapezoids before and it's much easier.

How about a cardboard template?