cutting arches

Hi Guys looking for a quick tip. I'm going to go ahead and cut the arches out of my disco 1. Their seams to be many custom aftermarket plastic/rubber arches on the market, to choose from. Do they flex to the new shape of the arch. Or should you purchases arches for a certain size cut? I intend to cut the arches out completely to the plastic inner arch but obviously want to make it look as neat as possible. Any tips on which arches sizes ect would be much appreciated. Cheers Jim

i would purchases arches fist, then cut to size

measure twice and cut once !!