This is my adapter to fit a GeekPi (PiBow type) case for a Raspberry Pi4B.

I very much like the simplicity of the Rainbow/PiBow tyoe cases with the multiple layers of acrylic stacked, but I wanted to attach one of the many HDD I have kicking around to the device so that I can keep them together as a unit when I configure it to run the OS from the HDD (well supported on the Pi4 too!).

This fits the bill well (even if I do say so myself), and uses M3 hardware to mount everything together after removing the bottom layers of the original case.

I designed it for, and have only printed it in, resin (Anycubic ABS-Like Resin+ on a Creality Halot One) so I don't know how it will fare on an FDM as I've not tried.

Edit: Note that the paint is still drying in the image….. it will be sanded back to look nice :)

Assembly was straight forward.

1 - Insert the captive nuts in the bottom (holes sized for an M3 nut)
2 - Align with the hdd/ssd and screw in place using 8mm M3 machine screw, making sure that the head recesses properly in the hole on the top of the adapter.
3 - Remove the bottom 2 layers of the layered case (designed around GeekPi variant) and apply over/around the Pi4B
4 - Align the Pi/GeekPi assembly over the hdd mount.
5 - Using the original case supplied machine screws pass these through the holes to align with the captive nuts and gently screw in place, being careful not to push the nuts out. 
6 - Apply some feet of your choice (I used some sticky backed rubber feet)
7 - Connect the HDD to the pi with a suitable USB adapter.
8 - Configure, plug in, switch on and enjoy :)

It is recommended in applying both sets of screws that you start all 4 loosely before tightening them all, and don't overtighten, as the acrylic can crack/break from being overtightened.