Search for "tire covers" on Amazon.com and you will find a large selection. Not sure how well any tire cover will survive under the truck but I am thinking of doing so to minimize weathering on the tire. You have to drop the spare to check the air pressure anyway unless you mount it valve stem down.
Yea that's what I'll do just get a soft cover, in Minnesota there is a lot of salt on the roads in the winter to counter the ice and snow. It can get ugly.