tooseevee -
Thank you for that information.
I know you provided a good picture showing the end result, but it would help me a lot if you had a closeup of the posterboard showing thickness and texture. I would also be interested in seeing how you connected the sides with the GG. I presume you ran a bead all along the edges and then assembled. How much flex is there in the completed "box" ?
"Pictures speak..." you know the rest.
I'll get some pictures later.
The poster board is standard stuff at Staples or office supply stores. It's about 5/16" thick. Very tough, shiny surface both sides meant to magic marker or paint on. If you find an opened package as I did - very cheap.
Can't show you pictures of building them. Measuring is the most important.
Cut your back & two sides to size, very square layout. Butt the sides up to the back & duct tape them together (like a hinge), the full length, at 90 degrees. Then lay the three pieces down flat. The hinge then gives you a space between the butts to lay a bead of GG. Then bring your sides up to 90 & prop them to dry overnight. Your duct tape must hold the butts together tightly to keep the GG from spreading the connections all out of whack.
Now you can lay your 3-sided box down on top of a new piece (open face down) & trace a perfect shape for your fourth side. Lay a bead on the two edges, have your duct tape already on, lay the piece onto its GG bead & bring your tape around completing the joint. Dry overnight.
Duct tape the whole length of the butts because the GG will expand & push the connection apart if you don't.
Now all you have to do is lay your 4-sided box on top of a new piece & pencil your cuts on the 4th piece on the INSIDE. Your bottom can then be GGed & fit inside the box. Tape these 4 butts also so the GG doesn't push the sides out. Dry overnight.
Next day you can add more GG where needed.
There is very little flex to these boxes. They're plenty tough; I'm not hauling bricks
I just left all the duct tape on & painted the top edges black so you don't see white with the flaps closed. Pictures later.