Bongo Fury

Hi folks,
I've been thinking of different ideas for modfing the inner roof of my aft Bongo.
I liked the catflap idea, reversing the roof panel etc as i can't afford to get it done professionally (that, and i enjoy making things!).
The more i think about it though, the more i am swaying towards the 3/4 roof bed option, where a 1/4 of the inner roof can be left open at night. As i have one child plus one on the way, i will be wanting something that i can access easily. I think the catflap hatch really doesn't work as well for this.
My question is what would i make the new inner roof out of? I assume the original roof won't cut down well enough to be used? Ideally, i'd like to chop up the original roof panel and use that, but can't see it being easy to strengthen?
So...any ideas?!

There are several versions it seems of the original Cat Flap idea some are listed here http://igmaynard.co.uk/bongo/forum/view ... 98#p565898

Plywood
12mm thickness should not bow
Comes in 8'x4' sheets

This might do the trick
http://igmaynard.co.uk/bongo/forum/view ... 97#p631397

Gasy,
I had initially thought of using ply but was worried that without being a box section, wouldn't take the weight of a couple of kids. If i were to use ply, i'd play it safe with 18mm at least (knowing how kids like to bounce!)

The type i am looking at is this
http://clearcutconversions.co.uk/produc ... terations/

It just looks the business for my needs.
All the others on thise threads seem to be centred around using the small hole still, which i ideally don't want to do.