Yep, lots of hatch doors on ebay. Just be watchful of the shipping costs. An awful lot of them have extreme shipping in my opinion. Most of them are overstock from rv mfgs, and are located in the midwest. I'm sure they buy them for pennies, then good markup, and more in shipping!!! I never saw a free shipping listing for a door. Small parts, yes.
I did see that there lots of plastic hatch doors for boats and such. They tend to be higher price for the larger ones, but a lot free or lower cost shipping. So I guess it kinda works out in the end. Either style, RV or marine, should require the same packaging and have equivalent weight.
As to framing, a print of the sidewall construction on our Timber Ridge shows that in the hatch area 2 of the "sides" of the hatch area are backed by alum, but the other 2 aren't. The screws just go into the lamination. Don't see a problem, unless a bowling ball gets loose in an empty storage compartment!!!