Dose your "Insta-Trunk" not go all the way to the floor behind the seat, and bolt to the fender wells?
I build my own insta trunk, as part of a modular storage system that I made for my YJ. Basically my trunk section can be mounted in two locations. In the rear location, the back seat can be installed, and it's just like a regular Insta-Trunk. With the truck part in the froward location, and the back seat removed, I can add a tonneau lid, and can secure all of the space behind the front seats.
Any way, sorry for getting off topic, here is a pic of how I modified a hinge to work as a quick relese attachment point for my trunk.
I took a couple stainless steel hinges, and removed the pin. I then bolted 1/2 of the hinge to the floor of the jeep, and welded the other 1/2 of the hinge to the trunk. When the trunk is in position, I put a cotter pin through the hinge, which basically locks the trunk to the floor. Also, these are located on the inside of the trunk, so even with the rear seat removed, the trunk is still secure.