Funny, I came to the same conclusion that there's just baaaaaaarely room for the motherboard inside the spare tire. Haven't done it yet, so I'm interested to see how yours goes.
I'm in the middle in installing my GPS and XM Satellite Radio antennas. (No WiFi for me.) I'll post pic's soon, but I actually mounted them under/inside the rear spoiler. There was less room than I first expected, but there was enough. Perfect reception (it's plastic) and completely hidden. I just need to route the wires inside and it'll be done.
Good luck to you.