Just an Idea for your 2hp left.

Either keep them open for later expansion of a small nifty utility you need to accomplish something specific. Or something more utilitarian like a passive LPG, Meng Qi - DPLPG comes to mind (the frontplate is 2hp but the vactrols on the back are a tiny bit lager, which could make positioning a little tricky in such a packed rack.) Also it's passive! So you spare some Amps for the other modules ;)