Harbor Freight had a sale over the weekend so I picked up their hot knife cutting tool for $20 to see how well it performs on cutting the plastic of the wingtip lens covers. The idea is that using a hot knife would not have the risk of cracking as a saw might.
I only attempted to cut the plastic lens in half and didn't try to refine it further to the final dimensions. I'm not sure that this method is necessarily the best option, since it leaves a large plastic ridge at the edge, though I suspect that this should be easily removable with a file or sandpaper. I may need to do some more tests on other sacrificial pieces of plexiglass to see how that goes.