To be effective the corner needs to be filled from bottom to top.
Are you saying you'd build a horizontal trap? That'd cover the resonance modes for the length of the room, and the ceiling to floor, but not the width. Imagine literal waves rolling back and forth between the parallel surfaces of the room. When you have a stack up and down in a corner of the room, waves going front to back, and side to side will come into contact with the most volume of the trap. But waves moving up and down will only catch it slightly. That's not to say that a horizontal trap doesn't help with the up to down mode. To control the two worst room modes though, you need two vertical stacks in two corners. You could put them in the front of the room.
To build a stack you really need the rigid board, so it can support 8' of it's own weight and not compress over time. If you built support shelves every foot, you could go with the flexible stuff, and not lose too much total volume over time.
