I have successfully used rare-earth magnets to hold some kits together. The most recent was a 'visible heart' model I did for my wife. I got some small (1/8" dia, IIRC) magnets, drilled small holes in some panels, epoxied them in just below the surface and covered them with putty and paint. You can't tell they're there, but the model doesn't fall apart at the least little bump.
You might be able to do something similar. I think I'd try to find a way to hide some magnets on the airframe on the plane, out of the way, but near your cutout, and perhaps get some thin sheet steel you can glue onto the panel, again at the edge of the panel, and get things to hold.
Carefully-cutting the panel away, and keeping the edges at 90-degrees may help, as could making thin styrene tabs you glue to the inside of the removable panel, carefully fitting it so the panel pops in place, but doesn't lock down so you can't remove it. Carefully-cutting panels, though, may require getting a couple of kits and using one for the cut out fuselage and the other for the cut out panels.
I see several ways to skin this cat. Hopefully you do a WIP on this forum!!
Gene Beaird,
Pearland, Texas