I replaced my plastic trim rings on my Lewmar
port lights, the ones you can't buy anymore. These rings are vacum formed I purchased a vacum table on the net that uses the vacum cleaner for the vacum. The plastic comes in pre cut sheets
.030" thick the same thickness as the original. First you have to make a form then you use the oven
in the kitchen to heat the plastic. The hardest part was trimming the excesws off. You can see one on the Deborah Lee's blog at Sailing Aboard The Deborah Lee
I'll gladly answer any questions.