Insulation did cross my mind a long while back but the thought of having it be dettachable, now that is a great idea. .
With regards to practicality, it can be done a number of ways since there are attachment points in a number of places- either to the sides themselves, the rear of the zipper flaps or hanging from the actual cross bars themselves with a simple velcro loop.
I think I like the last idea since it does not interfere with the current design and does not add bulk to the top. Perhaps they could double as blankets for camping assuming there is no hole for the windows... The top would be more difficult to line since it would require much more velcro. Perhaps the insulation just runs accross a straight plane from one horizontal cross bar to the other, leaving an attic space for the soft camping gear. Interesting idea.
Edit: if the top,were actually produced, velcro on the inside of the panels would be the best way to go since that design could be built right into the next version.