I didn't end up getting a track saw and I didn't make a jig, but I'll make one the next time I have a project. I tried just using a straight-edge and some measurement, and while the results are more than adequate for a hall closet, it wouldn't be for anything more.

We have a weird, triangular linen closet in our house, and the stock wire shelves just didn't fit it. Sheets and towels would just cascade over the edge of these shelves that only came out half-way into the closet. So I ditched those and just did the quickest job I could and still make it look passable. I cut up a bunch of triangular pieces of 3/4" plywood, sanded them down, glued half-round molding to the front edge, and set them on some 1x2 cleats attached to studs and toggle bolts.

I should have scribed them to the walls to make them a little tighter fit, but again it's good enough for this spot, and the result was almost a tripling of shelf space.