There's no room for the Opus, I believe. The dimensions are different than the M-series. Hell, your 3.5" HDD isn't going to fit in the Voom case, either. It's designed for a 2.5" laptop HDD.
As far as an M1ATX, I'm certain that you're going to be pushing it's power limits with the 3.5" HDD. It might work. It might not.