The basic procedure is to build out a lightweight wooden frame on the inside of the trailer that boxes in the curved door opening and makes a flat place to mount a screen door. Scamp uses nominal 1" stock for the frame. One side is cut to match the contour of the shell around the door. The top is just a flat piece.
Once you've accomplished that, you can use a small version of a traditional wood or aluminum screen door that swings inward or a bifold design like Scamp used for many years. The bifold design is not particularly decorative, but, as mentioned, has the advantage of folding flat out of the way against the closet when not in use.
Scamp does not make or sell the old bi-fold type since they recently switched to an outsourced sliding type. Even if they did, the contours of the frame would not fit your Surfside
Norm (Honda03842) built a version of Scamp's design from scratch, including the frame and an aluminum bi-fold door. You can find it somewhere in this very long thread:
Preparing a 1991 Scamp
The frame part will be similar even if you decide to go with a decorative wood door, which can range from simple…