Thanks for the suggestions everyone. I've avoided some major design errors with your help.
I ended up ordering a full set of 7U 110HP rails and endplates from Clicks and Clocks. Stefen sells beefy M3 rails with machined endplates for a really good price, and so I just got the whole set from him (including the odd Intellijel 1U format ends yay). I was happy to find the prices including shipping were very reasonable.
So for phase 2. I ordered a Intellijel TPS80W, it's almost 2x my power requirement. I am imagining I'll use a flying bus board cable for 6 of my lowest-power modules (maybe the attenuators and mults) since I only have 20 headers on the board. I got the matching power entry 1U tile from Intellijel, and I'm going to make a 1U-sized hole for it on the rear of the box so I don't have to do any actual wiring of electrics.
Once the parts arrive I'll start to refine the shape of the design to make sure everything fits right, but I have this so far:
See side view of box here: http://prntscr.com/k3d9gc
My main concern now is to make the internal size of the box large enough for the power supply and cables (and I've pushed this a bit towards skiff dimensions to match the side-profile of the Sub37). Deepest module is 50mm on the top row. Looking at my wood stock, I have some 1in cherry for the sides and some walnut from pallet-wood for the front and back.