Haven't tried what you did, but I have built a vocalbooth in my livingroom.. door and all..
Instead of buying expensive dampening foam from a musicstore, I found a lot cheaper foam at a carstore..
This:
It works great.. it's for carspeakers, the measurements: 40 x 60 x 2 cm.. and it's 5 of those pieces for 10$..
Covered my booth in it (still have a few places to cover at the floor), and I have no issues.. (yes, I've had experience with expensive foam)
I might have to try this out!