I used to have a load of small potting boxes, and I planned to use one for this module for a current project, but when I checked I only had a load of lids, and my supplier is out of stock.
The hollow in each lid is 4mm deep, so I put 2 back to back, with a 1mm thick plastic shim between, and managed to squeeze in a USB to TTL adaptor module, an Atmega 328 in a socket ( in case I had to change the sketch ) , a crystal , one resistor, four capacitors, and a Hope RFM83C transmitter.
If I forgo the IC socket on the next ones, I won't need the shim, it will all fit in 8mm.
The customer then said they dont want to use a USB extension lead, so I had to open it and remove the usb socket, and hardwire the extension lead. Here's a link to the other half of the RF link if anyone's interested http://i.imgur.com/vLpdcJW.jpg