Two part solution
I like them a little more quality than the originals or the RV solution.
So, cut out the shape that you want them out of a wood block, get a heat gun, some quality florescent light fixture covers from say Home Depot, and have at it. Works well. I have made a lot of these for a lot of boats.
Oh you are going to melt right through the first couple of tries...The trick is sagging, not melted, then pressed around the mold with another piece of wood.
Takes maybe 10 minutes max per lens.
Guy