I have had success in making small fiberglass parts using epoxy over foam method. I'd get a 4x4x1" sheet of closed cell pink foam from Homedepo (don't use open-cell styrofoam because it doesn't sand well),heat shape it to the gentle curve of the front of your trailer, then sand in a nice radius all around using a long sanding block. Then lay-on 3 layers of 6oz cloth orientated 45 degrees from vertical. You must use epoxy because boat resin eats styrofoam. After curing, trim the edges and dig out the foam.........or, just leave it there, except where the hinges and handle goes. Fill and paint.........seems like I remember having a red door on my Eberly.
Have fun,
JJ