Bluemike. Another option for making a panel is as follows: Make a thin wood frame with the dimensions of the frame members being approx. 1 to 1-1/2" wide and 3/4 to 1" thick. I lap jointed the corners, but mitre cuts would work too. Build a frame with the outside dimension being 24"x 48" up to 36"x 48" Three pieces of rigid insulation will make a total of the two larger panels. I like the appearnce of a golden rectangle better which is 30"x 48", but it wastes material.
Once you have the frame, run a bead of glue around the front surface of it and mount the rigid fiberglass. The frame gives some lightweight support to the panel, but more importantly, it moves the panel off the surface of the wall (3/4" to 1") and makes the panel more effective. (I think Ethan's site explains why) With the frame behind it instead of around it, you also get the increased surface area that comes from the exposed sides which increases the effectiveness.
Since you don't want exposed fiberglass, you need to cover it with some sonically tranparent material like burlap (cheap) or Guildford of Maine (quite expensive) That can be stretched tight around the fiberglass and frame and stapled on the back.
I used 2" thick Owens Cornig 703 for the first reflection points which works really well. It comes five 2x4' sheets to a pack or you can order individual pieces from some mail-order companies.