Bonnie , just thinking a screen door latch the locking kind with the spring and slider ... Simple , inexpensive , available most everywhere...
If you used Lexan for the windows , that’s stuff's bulletproof ... although somewhat pricy ... Outside pane would be easy ... and like you say , keep the rain off . The magnetic screen on the inside is easily removable and keeps the bugs out .. Round window cutout best supports all stress loads and needs no reinforcing ... just cut a hole ... You could even save the cut out as a plug ...for Winter storage or just privacy ...
For WET areas like ours

, a good drip edge overhead works really well on most every door or window ... !!!

On a Foamie , that’s easy ... made of foam ...
