Just a bit of an update, I've been working on the frame and fleshing out various details of the case, nearly ready to get the first 3D prints made.
A first draft of the hard drive cage can be seen in the screenshots below, as the cages are to be 3D printed I can offer different configurations depending on how many drives you want to install.
The cages are designed so the drives slide into the cage and clip into place without screws. Each bay can take a 2.5" drive from 7mm up to 12.7mm. I could also offer a 15mm version if there was interest.
Single Drive version:
[MEDIA=imgur]jZlGSUl[/MEDIA]
Double Drive version:
[MEDIA=imgur]LrHU5i6[/MEDIA]
A triple drive bay could be designed, but I think the maximum drive thickness would need to be 10mm.
The case pictured above is the taller version which can accept full height cards with the power connector facing upwards. I've decided to put the power/reset buttons and USB ports on the side of the case next to the intake fan, this keeps the front cleaner looking, it has also given more space for the hard drives along the front of the case.