We don't want to discourage you from making these bass traps, simply because of the woodworking that's needed. But having even a minimal structure inside these to keep them from flopping over, will help you considerably.
It really could be built with the cheapest and simplest of wood material, and some simple screws... all the way up to a nail gun and glued joints with corner braces.
I would start out simple, and find some scraps of wood trim from someone, or go buy two long sections of unfinished pine baseboards. Some 1/4" particle board triangles used as corner braces will keep things square as well. It doesn't need heavy duty hardware, it just needs to be held together. Then, this frame work will let you staple gun the fabric around your pieces of insulation.