Seems like a lot of work... But then a silly, but maybe actually brilliant, idea struck me...
Why not turn my pop into a high top, ie. raise it up then fill in the gap with some sides? Then somehow stick it all together... Perhaps even turn it into a solid sided pop (a bit like the autohomes one?) where the sides fold in?
The idea is so that when its up, its strong and air/watertight enough to be driven just like a hi top, but it can be folded down... I would probably want to angle it a bit too, maybe something like this (apols for nasty photoshop jobby)

I've actually got a Devon, this aint one, but you get the idea (hopefully) not too sure about doubling up the rear panel just wanted to see how it would look, would probably just put a porthole or small sliding window in it.
My reasoning behind this madness is that its probably easier and cheaper to make a couple of straight sides than it is to make a whole high top from scratch. Buying a proper high top of course is an option, but just an expensive one. If my idea can be done cheaply but also result in something which worked well and didn't look totally hideous then why not eh?