Why aren't there aftermarket battery covers?

I'd pay 40$ for a nice glass and metal back door if it didn't affect the phones profile at all. With a lot of people sticking with the note 4 I assume a lot of them would also be willing to spruce it up a bit. I've found one shady eBay seller with metal back doors but nothing else, it seems like an easy accessory to make that anyone who keeps their phone naked would like.