I installed hatches in my 19' Scamp back in 2007. At that time what I found and went with was brand line called Nuova Rade from West Marine
I went with the 14 x 24 inch size. That was a squeeze for my 19' Scamp. BUT the resulting storage was amazing! (See pix below. And note: my mods included moving the water tank and other reconfiguration changes to maximize storage space.)
For a 16' Scamp that same size hatch would work equally well. Both share the same lower shell after all.
For a 13' Scamp, however, the wheels encroach more under the dinette benches. That leaves less flat wall space to work with. So smaller hatches are needed for smaller eggs. Hold that thought!
Meanwhile, here's some follow-up field experience. In hindsight I didn't need the hatch opening to be quite that large. At the time my requirement was to carry my portable gas grill back there. And that worked great. Isn't there always a BUT?
Drawing such a large hatch down to the shell did torque the hatch frame. That puts more stress on the rear latch. After 6 years it's not sealing 100% anymore. A solvable problem but still. If I had it to do over, I'd choose a smaller hatch that would fit better. (I stopped carrying my grill in the Scamp after making an even better place for it in my truck bed.)
Coming soon: a MUCH better hatch solution, especially for 13' Scamps!