aluminum solder is shit, sand it flat, use the proper aluminum solder and ultra smelly agressive flux and scrape it with a sacrificail iron to get some kind of wetting, then sand the nasty oxide off both tinned surfaces, hold them close, reflux, remelt, should hold (poorly), getting it to 'flow' is like convincing a cat to do something (maybe dripping hydrochloric acid on it and washing would work well )
I never got it to 'look nice' its like a bad weld job where you need to sand it afterwards
I kind of wonder how a rivet would work if you put a star washer for it to compress against , something like pop rivet, crimp, star washer, aluminum
flux matters:
https://superiorflux.com/portfolio-item/tensile-strength-testing-3003-aluminum-soldered-lap-joint/what I like doing with aluminum braze is low strenght frames for filters to put over chassis holes. you glue window screen material into an aluminum frame (you can cut the aluminum flat bar with bolt cutters, arrange it in a shape on a fire brick and braze/solder it together very quickly, since you are putting glue and filter material over it, it does not need to look nice, and materials use is minimal since you are not cutting holes in sheets etc use thicker flatbar so the edge joint has some strenght and cut it on 45 degrees, but I am setup to do this quick with a proxxon mini-saw
i put a rubber gasket under neath I cut out of things like foormat or toolbox mat liner material around the frame.
obviously not a EMI screen