That would work too. For now, a large rubber band solves the problem. But it would be easy to move the retaining pieces back a few mm for a more secure fit without the need for Velcro or rubber bands.
On second thought, Velcro might not work since the top device slots in like a NES cartridge. I’m also not sure which device should sit where. I swapped the positions of the MD and MNM because the curved sides interfered with the MD’s knobs but not the MNM’s buttons. I’ll need to jam on the Behringers before I make any further design changes.