The bottom hatch is retained by having a poly-ply tongue on the front of the hatch, which slides up inside the fuselage, then four screws that go into plywood tabs which you attach to the fuselage.
You then apply poly-ply to the inside of the fuselage, at the rear edge of the hatch. This helps the hatch maintain its curved shape. I got a bit carried away and put poly ply on the sides too - which is not called for.
When it comes to gluing material like poly-ply I use a special version of CA that is designed for use with composites. In this case I'm using
PIC Plasti-Stic. There are other CA's for plastic (etc) too, but I particularly like the PIC version. It's a MIL spec glue, which does not seem to be advertised to the modelling community (mostly military / industrial use apparently), but I'm currently trying to get the manufacturer to send some of his product to RCU for review so that more of us modellers can hear about it.