Screening and nettings work to some extent. The no-see-up netting I put
on all my portlights a few years ago is now degrading and falling
apart. Bummer... that was a lot of work. I keep PIC coils on board for
when it really gets bad. Those are the little green coils that smolder
for hours, releasing something that both repels them and kills any that
got inside. I bought a metal holder that allows them to be placed
anywhere, even hung from the overhead. Probably not the solution for
all times, but really works when things get bad. You can find them at
pretty much any camping store, or on the web I'm sure.