There is a lot of us looking at building piko satellites, even if launching cost currently is prohibitive (Interorbital with their Tubesat might change that). It is fun to see what one can do within these small formats, but still, there is a lot of cut fingers, making PCB boards that fits.
Why not look at making a series of PCBs that is adapted to these three formats. A square PCB fitting in a 100x100x100 mm Cubesat (in principle a cut of 160x100), a round fitting in a ø86 mm tube as well as solar PCBs 26x120 mm.
PocketQube (1/4 of a Cubesat 50x50x50 mm) have a totally own design, hard to carve out and a pain finding M1 nuts and bolts as well as some “corners” to assembel ones Qube. The PCBs is also its body, so that is really is a kit.
We also have NASA/ESAs CanSat challenge for High Schools, also in business for PCBs fitting through the opened up top of a 33 cl soda can.
But I am sure some of your vendors can both fix this with an ease.
Also, they would be able to fix good solar cells to fit. Right now I use some Vimun SC-3722-9 I got from a sale of small solar flashlights at just above $1 a piece. On the heavy side, but as I can judge reasonable performance. But panels 45x90 mm for Cubesats, one 24x100 mm for Tubesat and 46x46 mm for PocketQube would be better.
Cheers