I came up a a good way to lay out the slots for traditional rear axle plates. I didn't take step-by-step pics, but I think I can describe it well enough...

I am using 1/2" plate for the axle flanges, and my frame tubes are 1 1/4". I wanted the axle plates centered on the frame tubes, which meant I needed to lay out cut lines 3/8" in from each edge.

I clamped a large plate to the inside of the frame rails, to the inside edge where the tire would go. I used a piece of 3/8" scrap as a spacer from the plate to scribe a line on each tube, then I stacked a 1/2" spacer on the 3/8" spacer and scribed the second like.

A few quick cuts with a cutoff wheel on my right angle grinder, and 15 minutes of test fitting and adjusting using a 2" right angle sander and a file and I had a pretty nice fit.

(note the plates are just roughed out here, I need to fine tune a few things yet)

one more...