I'll plug Doug Fattic's sizing fixture (or whatever he calls it). It does not have any inherent flatness. It sits on a flat surface to achieve the planerity (if that's a word).

To further focus on this idea. I make little brazing holding fixtures as i need them. One of the design goals i strive for is NOT having to make them with precise machining or surfaces.

I find the efforts spent on one's jig being precise is lost when you braze with poor or uneven heat control. All a "straight" jig will do is reduce the amount of cold setting after tacking and before fully brazing.

To the OP's jig- one element I picked up on is the rear axle that looks to have no ability to fit drop outs of different thicknesses. Andy.