Making custom joiners

I’m wanting my 104hp 7U case to sit vertically on top of my MS-20 mini. I bought the joiners to see how things fit together. I could find no specs or tech drawings on the shape of the end of the joiners. As far as I can figure out I’ll have to get some 3mm Aluminium sheet laser cut (then powder coated?). I hope this would be stable. If this works maybe looking to get another 7U case to attach vertically on top in the future (with other custom made joiners). Is there any further info out there?