I believe a normal weatherproof junction box should work fine (they come in all sorts and sizes! i know i used to love going to home depot as a field service egineer, i installed plenty of them)
They are magnetic sensors so visuals should not be a factor. As long as they are no more than 1/2" from each other and are pointed towards each other properly as setup instructions say to do, then there should be absolutely no problem weatherproofing, given you make measurements twice and cut once, you should have no problem and a dandy awesome new outdoor project. Be forwarned though, since you modify the equipment, Comcast can only support it if you appropriately use the equipment as they instruct. Comcast do not own your sensors so you can do what you may with them.
Excellent idea for a new sensor market though!