I have used the lightweight spackling material in the past, but I am open to different methods. I just want it really light.

For the hatch, I added a balsa former to the front and rear, then used some balsa stick to reinforce it. The balas sticks sit on more balas sticks glued to the kit's plywood formers. I have not decided on how I am going to secure it, but right now I might use music wire pushed thru holes drilled in the firewall thru the hatch and then into the next bulkhead or I might add a couple dowels that go into the fire wall with a hatch release similar to what the jet guys are using.

I will try to get some more pictures of it for you.

The hardest part was finding where to cut the hatch out, as I had sheeted the entire area after adding the extra support material, then carefully probed for the corners, then used the exacto to connect the dots.