Sorry, just realized I haven’t relied to this post. To answer your question, unfortunately I don’t have a copy of the case design files — the case is designed by SeeedStudio and they contracted it to a Chinese company to have the cases manufactured. Given that there is plenty of space close to the top of the PCB, you should be able to easily fit an nRF24 module there. If you want to use nRF24 with antenna, all you need to do is to drill a hole on the enclosure to let the antenna pass through.