I could not find a suitable solution online, and seeing the price of a simple hard drive holder online made my poor wallet run away.
So this is the solution that I came up with!
It is stackable, using m3 screws and inserts, although they are quite tricky to get in properly, and I want to improve the design for easier screwdriver access.
It is technically compatible with 2.5" drives, but I am working on a better way to hold them in place.