Poppy,
My convoy above cab area has been turned into a cupboard, and it hadn't been insulated either. This was then getting condensation in it due to the lack of insulation, making the van colder and the cupboard about as much use as a chocolate welding torch.
It also meant that getting at it to insulate it was a royal pain!
Anyway - as it is a cupboard, so the aesthetics of the thing was secondary to the function (plus being a pain to get at) what I did was to buy some 3-season camping mats (3 of was the right amount btw) and simply cut and glued them to the glassfibre roof to cover it. Seems to have done the trick. I was going to then glue silver insulation foil stuff over that as well, but not sure I need to bother.
BTW - if you have a branch of The Range near you, have a look in there for the adhesive glue spray stuff - its £2.99 a tin in there - way cheaper than I've seen it anywhere else.
Think I did post something about this in my Sully the LDV Convoy thread as well :goodluck:
EDIT / ADDENDUM: If you don't want to do that, then best guess is to Sikkaflex glue some wooden ribs to the glassfibre roof, and then insulate and cover as you've done with the rest of the van? ou might need to get creative with a wood plane to shape timbers to get a good fit to the roof curves and a good bond for the Sikkaflex though first? This is just guesswork though - surely there'll be a home-build guru along soon though to give a definitive reply!