The first thing we did was try to figure out what size hole to cut. This was not easy! In the end we just sort of guessed and cut a hole in an old bit of plywood. We were wrong, so it was lucky we had practiced first! We tried again and eventually got it right. We also cut out some celotex to line the window to roughly the right depth, then mounted the window to check it would close properly.
Next we cut a big hole in the van!
Then we cut out a bit of celotex to the right shape and glued it in place.
Then we ply lined the top part of the door. This was a bit tricky as the depth of the window was more at the bottom than the top and we had to pad it a bit.
We realised after we did this that a wooden frame would have been better on the inside of the window, for structure and to attach some kind of closing mechanism, so we had to cut out a bit on the celotex and shove some battens in. (When we did the next window we did this properly).
To make sure water doesn't get through and run down inside the ply and insulation we used damp proof membrane overlapping to the outside by about 2cm, which is very sticky and malleable, and hopefully waterproof! I am a bit worried it might melt in intense heat though.
The window now closes and doesn't seem to leak. The closing mechanism (garden wire and screws) is not really pretty, but it works for now.
We did the same for the second window, but it took about half the time because we knew what we were doing! The closing mechanism is different, we screwed 2 L shaped brackets into the sill.
For the roof vent we got a proper Fiamma 40cm square vent. (about £50).
We cut a 39cm square hole as instructed. We tried to even out the ridges on the roof with some pliers. We made a frame for the inside using 25x 40mm battens (the depth is 40mm, which is the maximum depth the vent can be, and about the minimum the ply lining and insulation can be). We used the sticky damp proof membrane folded up several times, to act as a seal, both on the vent and around the hole. We screwed the vent on to the frame really tight.
In the end we bought some Plumbers Mait putty and shoved that in under the gap. We also pushed the van roof up a bit so the water doesn't pool as much. We sealed around the edges with all weather clear tape and this is now waterproof (fingers crossed)!