This is a good idea.My GPS is mounted in a bracket on the dash, no pouch or heater. I have a small digital thermometer mounted just below it on the same bracket. Once the sled is warmed up the temp usually stays around 40 - 45 degrees except on the coldest days (like 0 F or below). I don't leave the GPS there overnight or during meal breaks. I haven't had any problems with the cold affecting the GPS operation.
Did it work well for you last season?here is what I have come up with. Case cost me $4, hand warmer cost was $15.