MrSheltie. If I understand you correctly, you took a mini truck camper shell, installed a floor in it and hoisted it onto your van roof and bolted it down. Is that correct? Do you have any pics? Sounds interesting.
No, it was bolted to a pipe rack that fixed to the rain gutters.
You could have it off the van in 10 minutes.
It was STRICTLY to sleep in, by no means should you think it was a weekender, but it did solve the issue and would sleep two warmly without taking up space in the cargo area.
Having sold it, I am trying to get you pictures...but it was nothing more than a cap with a plywood bottom, then the plywood was bolted to the pipe rack with reversed U bolts.
The back glass closed against the plywood and the thing worked pretty ****ed nice. (Watch too much weight in it though...but for sleeping bags, clothing and pillows it was fine.)
In one of the older vans, (high tops) over the driver/passenger is another place I have seen people make a bunk. Has to be a VERY tall roof though.