I have your solution for a MC184LL/A apple wireless keyboard battery cap. I was a bit annoyed at the fact that there were no spare parts to be found, and paid the 80 bucks for my Wireless blue tooth slim keyboard jumpoff, and told to buy a new one. Somehow lost the battery cap, and didn't want to pay out again, so here you go.
Go to Your favorite hardware store's, Hardware Section, your looking for their 2" long, 5/8" thick Hex Screws probably at the bottom with other larger screws. should cost $1. 9/16" is the technical width of the apple battery cap, great size choice apple, but 5/8" screw works for my solution.
Tinfoil about 1"x3" wide strip fold up enough so it will fit inside battery compartment.
Put in the 3 batteries flat side up, put the piece of tinfoil on the last battery, screw in the hex screw till its snug, press power, you have a working bluetooth keyboard. (Looks frankensteinesque) So if you have a metal saw, you could trim it down a bit and make your own more ergonomic caps.
Until I get a metal saw, Im working with this: (see picture)
http://gallery.me.com/vtmhealy#100014/Keyboard
No Pretty but it works.