This is a simple, modular stack system for the Raspberry Pi 4.

I've made a tray module for 2.5" USB HDD cases. It should be compatible with most models. I've used the Orico 2588US3-V1. You can use double-sided tape and few strips of soft foam to prevent the HDD case from slipping out and also avoid scratching it.

The tray for the Pi itself is compatible with the Aluminum Armour Case and uses the 4 screws of the case on the bottom for mounting. You can also use the fan-version of the heatsink as well.

The third tray (without the stacking posts) can be put on the top to close the stack and give it some more rigidity.