These trays each hold a Raspberry Pi which can be attached by M2.5x4 screws with an optional fan mount - just use longer screws (M2.5x16) to attach both. The fan mount could also be used on its own and fits 30mm and 40mm fans. The trays are stackable and hold with a light friction in another. They appear a bit flimsy when printed but once mounted to a Raspberry Pi with all four screws they become solid. The holes are tight enough that no nuts are needed for the screws.

Edit: Added a bottom part with short legs