Depends somehwat on thier purpose. Some may just provide a cosmetic cover, others may be structural. For cosmetic covers, very little is required to hold them on. Even tape works to hold a cover on. However, if the cover is structural, you need to take any loads into consideration.

What I typically do is create a tab or pin and socket at one end and then have a fastener of sone type at the other end. A tab would typically slide under some structural item. A pin and socket could be a simple as a small dowl and a drilled hole. For fasteners, all sorts of stuff will work, I've used some spring loaded captive locks before, but I typically just use a screw of some type. Sometimes with a blind nut, or insert, or even threaded wood. There is no reason why you couldn't use screws all around, but I find it easier to use the panel if I have less screws to mess with. As for material, again it depends. Balsa is a little flimsy for something that will get some abuse, so I typically make my own plywood or composite. Easy to do, just sheet 1/16" balsa with fiberglass or 1/64" aircraft ply. The final result is a very thin and light hatch, but stiff. You can also laminate over a curved surface, such as a mailing tube to create a curved hatch. You can also make a fiberglass (or other composite like carbon fiber) hatch fairly easily. But there is no reason why you can't use light ply or aircraft ply.