I too wondered about the air intake for the box? I used to have a rig kinda like that for my HM alone, I found myself leaving the cover off so I could see and adjust the HM easier... Murphy's law eventually kicked in and the box filled with rain! LOL
I have left my RD3 out on my grill 24/7 literally for years now completely unprotected with no issues, you just need to orient it so the rain runs off rather than running into the probes and cat5 jack. From there, thanks to the handy RDTC board the CAT5 cable allows the HM to sit safely in my kitchen, or somewhere else that is safe from the rain. Added benefit, I don't have to break down the rig after a cook, just set the HM to OFF and get right to the important work of enjoying some BBQ!