I think it would be better to add drives mount somewhere in front. Ideally, drives should be separated away from GPU & CPU heat.
Awesomely drawn diagrams below, based on OP.s latest render. Looking at the case from the GPU side. "|" are dividers.
Back <---> Front
GPU | PSU | drives
GPU to PSU divider can be made from acrylic plate, hell even a thin film of plastic. Enough to block the heat away.
The drives can then be attached to the same divider holding the PSU (this one needs to be sturdier then, like metal), or to the case's front cover.
However, not sure if it's still gonna be < 5L, and not sure how that translates into production cost / complexity.
My 2c.