I just got the cheapest 1 x 3 boards I could at Lowe's. I made sure all the boards were straight, which took some time. I had to go through quite a bit of lumber. For bass traps, I used 1 x 4 boards.
They didn't end up being very heavy at all. I was actually kind of surprised. Panda's design is really good because it leaves the sides open. However, it's more labor intensive. After you put your frames together (or before), you could always try routing out the sides. That might work.
My end peices were 2'. I forgot the exact length of the sides, but all you have to do is lay the board next to a sheet of 703 and mark your lengths. Then just cut all you need at those lengths. That worked great for me. The 703 doesn't have much varation on size...it's pretty consistent. I put my frames together with some wood screws. Two in each end. I also used picture hanging wire and eye screws to hold the 703 in place, which left me a 1" air space behind the 703.