I have a large overhang off the back with plenty of room where a spare tire should have been mounted. I went the cheap/easy route. I got two ratchet straps and strapped the spare tire between the two frame beams. I was able to loop the strap around the frame and floor. I followed it up with a length of chain in the event of a strap failing. I will still be dragging a tire with a strap failure, but the tire won't go flying off.
Have used the spare once in two years, picked up a screw at a campsite. Hardest part is putting the tire up, gravity sure makes it easy coming down, just don't be below it!