I have not done it but have looked at a few options:
One option is to build a single trapezoidal frame that drops in your crib board slot. The downside is you have to remove the whole thing to get in and out. Storage will be interesting...
Another option is to replicate your crib boards and put a large opening in each board covered with screen. Again, you have to lift them out, but it is in smaller chunks. Easier to store...
A third option is to use some sunbrella or other UV and environmentally stable fabrics and sew up something with open netting fabric in the middle. Think tent window. Use snaps to hold it in place. Downside is if your woodwork is freshly varnished, it is a very painful thing to install new snap hardware..... YOu could install snaps inside though to not mar your brightwork. This third option is going to be the best storage-wise.
As for the hatch, I personally would sew up a simple netting screen, and I would use magnets to hold it in place. You should be able to find some of the little rare earth, (neodymium) magnets for this. Sew a few into the screen, and then epoxy mates to your cockpit inside the hatch. Many might suggest velcro, and it would be a good option as long as you get the non-adhesive kind and glue it up on your own. PSA backed velcro will eventually fail on you, leaving a gooey mess....
As I type my recommendation for the hatch, I come up with another interesting idea for your companionway screen. If you can get the little rare earth magnets, consider this:
Remove your crib board wooden rails. From the backside, counter-bore shallow holes that will just accept the little round rare earth magnets and epoxy them in. 3 or 4 per rail should do it. The magnets should be flush to not interfere with your crib boards when installed.
Now for the screen, first manufacture two "frame rails" that will fit the companionway hatch slots, one on each side. Mark, drill, and install matching magnets in your frame rails, (observing magnetic polarity) so that when installed the magnets line up and hold the two rails in place. Now sew up a screen panel with rail pockets on both sides and insert the new rails.
A device like this will stow easily, as you just will roll it up. It is VERY easy to get in and out with the screen in place; just remove a rail, crawl in or out and then reinstall. the magnets should be strong enough to provide a very satisfying "snap" when re-engaging... It also gets you all the air that the thrid option above would provide, without screwing up your brightwork with snaps hardware......
