We have a screen on our pilot 30 that works wonderfully. The screen is wider than the opening with wood bars spaced about every 24". The ends of the bars are rabbeted. Stows away neatly while attached forward of the opening.
This weekend I could get a picture.
Greg
I made a screen for the cabin door of my last boat and it worked great. I just bought some regular fiberglass screen material cut it to be about 1" larger than the opening and a length of a 1" wide heavy duty ribbon long enough to go completely around the edge of the screen.
I had an OGF (old girl friend) who had a sewing machine stitch the heavy ribbon around the edges and stitch some Velcro pieces (2" long, spaced about 24" apart) to the ribbon.
I put up some Velcro on the boat in the corresponding locations to match the screen to.
All it takes is to unroll the screen and match up the Velcro pieces. Worked great and when rolled up was easy to store.
Never owned a Mainship, but for various other boats, both sail and power, I've done similar. Using the same flexible fiberglass screen as seen above, I sewed up fabric edging tape around the perimeter as GFC mentions. Rather than use Velcro, which ages quite quickly on a boat, I used brass snaps. Works beautifully.
Besides screens, I use a couple of bug light zappers. I like to anchor in remote places and often stay for more than a month. After a week or two I notice a noticeable difference in the number of bugs. When the lights are hung over the water, at night they attract fish that eat the falling dead bugs and are still around in the daytime for breakfast.