We lost our porch light lense somewhere on the Trans Canada Highway. I cut the bottom off a 2-liter soda bottle. Carefully twist the end of a sharp knife through the side to make two holes that line up with the pegs on the bottom side of the fixture. Pop it in. Ours stayed in even on the Top of the World Highway in Alaska.
We should be able to drive down any highway and find these things lying all over the place.