I got the lower frame tubes (we called them chain stays when I worked on bicycles?) done first to position the axle plates, then made the top tubes (seat stays) fit. When I bent the tubes, I left them long. I was about to cut the front section to size when I realized it would be easier to find 90 degrees with the long section still there. So, I cut the rear section to length and slotted the ends for the axle plates before I cut off the front section to fit the frame.
Once the front of the tubes were cut to length, I discovered that the bender deformed them more than I had expected - they fit pretty loosely in the holes now when in position! I ended up making a sleeve that would fit between the frame brakets and slide over the frame tubing. I slotted it so I could see the tubes inside and then welded everything together once it was positioned.
I don't have a mill but I do have a lathe, along with some tooling, so I put an end mill in the lathe, fit up a vise, and cut the slots in the chain stays. The set up isn't nearly as rigid as it should be, but for what I'm doing it works as well as any other method I would use.
Next I decided to work on axle plates. I drew up some ideas that night and worked out a few issues that I found. Finally I settled on the designs shown. They aren't the same because the left side has to incorporate an anchor for the brake plate. That's what the slot in the front is for - a bar will be welded in there, which will fit into a slot on the hub when it's installed and keep the brake plate from turning when the brakes are applied. I also left a tab on top for the fender mount.
I slotted the axle plate the same way I did the tubes, it just took longer! Actually that isn't entirely true. The slot for the axle had to be 0.60" (Yeah, I should be stating that in mm but my tooling is in inches, so...) and I didn't have an end mill that size, and didn't want to be cutting that much metal anyway with my hack machining set up, so I first drilled a 0.50" hole where the end of the slot needed to be, then cut a 0.50" slot out on a bandsaw. Once that was done, I cut the slot to size with an endmill in the lathe, dropping one surface and raising the other in two separate passes.