chemie,
I think maybe you are "over thinking" the hatch. I agree with you, it seems a bit odd, but I built mine per the instructions and then sanded it down while mounted to the plane. In fact, I sanded one side, then used a razor plane on the other side to remove most of the wood, and then finished sanded it. I does come out just fine. Maybe contact Sig like SeamusG suggested?
But since you brought up the hatch, I was curious how most people installed the canopy. I useally use smalll canopy screws, but not sure about this one. Was going to paint the interior, and then maybe glue to the painted surface. What do you guys think? And chemie, sorry for the minor hijack, but you will come to this part as well, if you haven't already.

Oh and "natural birth" remark made my day, LOL.