Sorry I took so long to respond, I just saw your post. I had a box made out of vinyl snowmobile seat cover material, the box is large enough to line with 1" foam. The top is clear vinyl and velcroed to the vinyl box, a strap is also attached so I can carry it around my neck to free up my hands.
I use hand warmers to heat the GPSr, a slot is cut into the side for external antenna and enough room in the box for a lightweight rechargeable battery pack. This works in -40° temperature.
I hope this helps I'm kind of late responding.