Yes, agreed, you need to use glue. I used the supplied spray glue which worked well initially but is now coming unstuck. Mine's obviously a monocoque, but used one piece over tunnel top and sides, one piece each side wall round to front footwell and one piece for boot floor and down behind seat backs. Floors each side are plywood and have a rubber mat under driver's feet to stop them slipping on the wood. Mat needs to be stuck down or it rucks up. Gear/brake levers gaiters are supplied vinyl cones and trims are cut from alloy sheet. Not sure if this helps or is relevant to tubies?
Here's a picture if it's any use: Cockpit