Let me explain: I have repaired many First Generation Nexus 7 tablets, and they will power on without the back casing...

I have Back covers, and other replacement parts for the Nexus 7 on my Ebay page. I think I only have Two backs right now, but I always have one or two on hand. Sell one, and its replaced by one. I don't know if you can search Ebay by seller name, but my handle is snakebitemagee, or just search ebay for Nexus 7 case back