Tho a closet full of clothes will do the job, and I have used that for many years along with moving blankets behind me
, the newer reflection filters do more than absorb high end. The better ones can absorb to a surprisingly low midrange area.
And the one thing to remember is these type of shields are NOT for sound isolation for the voice or cab that you are micing, they are to cut down the Room Reflections from getting into the mic in the first place......a different animal all together.
Whisper Rooms are a product that can give a pro voice over artist the full treatment.....for a price
. There are tons of other companies out there that sell similar products, but I only have experienced with the 3.5x3.5 & 4x6 Whisper Rooms.
Good Luck in your search!