I agree with Sonic Misfit, the plexi glass does reflect the sound too harshly - and candidly, I have never found to effectively isolate all that well. However, I don't know if I agree with sound absorbsion.
If you are close micing and don't care about a room sound, then maybe surrounding the drums with baffels may work. I've always felt drums (or any accoustic instrument) is as much about room sound (maybe more) than the close mic sound.
However, if you do want to isolate the drums - a cheap way is to buy used draperies and hang them around the drums.
Or, you can build baffels by making a frame of 2"x4" boards and tacking a materia (burlap as an example) to the frame.