In the "standard" covers, you can get the basic plastic or an upgraded (Lexan?) version. I believe Fantastic provides only the Lexan type. Also that they furnish replacements free for some of their parts, and I thought covers were one of those.
There are many good reasons to use Vent Covers. MaXXair is a common brand and the kind I buy. One of those reasons is they serve as a first line of defense against hail, tree branch, etc. damage to the actual Covers. Not to mention being able to open Covers on the road and in the rain.