You can buy the OceanAir stuff, which costs about as much as the whole hatch. Or if you don't need it to be quite so posh, you can do what we did:
Buy a roll of adhesive backed hook tape (e.g. Velcro - hook and loop but only need the hook side). Stick all around on the inside of the frame, which on ours was the perfect width to do so (Gebo hatches, but I suspect Bavaria used the same inner trim). Now buy a pack of household mosquito net for a few coins from the supermarket. Stick it onto the hook tape, which will hold it securely, starting in a corner and working your way around. Don't tension it too much, just enough to not flop around. Cut off the overhang - leave one corner a bit longer or sew on a little bit of webbing so you can pull on it to open the net and access the hatch handles.
Costs under €5 per hatch. The mosquito net gets UV damaged from the sun and needs replacing every 1-2 years, at an additional cost of €2 for the entire boat. Made a big difference to our sleep quality!