If the problem is limited to the trim and the basic structure remains
sound, I would make the following set of guesses:
The trim is not painted on all faces before installation.
(This includes the cut ends.)
The trim is not sealed to the body of the bay or the sealant is not
maintained.
The trim is not shaped to shed water.

I would * not * wrap the trim.

There are a number of synthetic materials that one might use.
The first that comes to mind is fiber-cement.

Tom Baker