Do things the Seidio way. Basically, make a new bottom cover for the MBPR, made from aluminum billet, with a different taper, so that the MBPR sits ~10 mm taller. (Edit: Nope, not extremely thin. I fail at math, clearly. It's thicker than the MBP if you do this.) Put two things in that bottom cover. The first thing is screwholes for a 2.5" HDD, and a SATA connector with cabling to go to the SSD slot on the motherboard. The second thing... big-ass battery. And I mean huge. I think you could get 12 cells with the same capacity per cell as stock, or 6 double-capacity cells, without even trying too hard. Then, either screw this into your bottom cover, or make a retaining strap of sorts to screw it to the unibody, then screw your bottom cover to the retaining strap instead of the unibody. Sell the bottom cover with SATA connector+cabling only for $100-150. Sell the big ass battery for $250, and offer a bottom cover, SATA connector+cabling, and big ass battery for $300-350. Add another $50 a service where you can send your MBPR in for careful removal of the old battery. Any accessory maker that wants this idea, it's yours, for free, and it fixes every problem but ethernet (you've got space to do it, but the ways to do it are ugly and hackish), optical drive (you could actually fix that, but it's also ugly and hackish), and RAM (no way to do that).