Welcome to FGRV Pam,
This is more than you asked for, but the 1300 is very well suited to what many call the screen door mod. Oft cited is Gord100's description and plans that seemed to disappear from the web for a while. The good news is, I just found out where his website has moved to:
Gord100's Trillium Trailer Webpage
Here is another nice example:
Screen door for my 1975 1300 Trillium
Some people choose to install a folding screen door like on the Scamp
rather than deal with the single large door.
If you don't want a door - the frame works better for your original plan since the body curves out from where the screen will hang and you will end up with big gaps.