In my studio, I use 2'x4'x4" slabs of mineral wool wrapped in acoustically transparent fabric (can blow through it). I've spent a little more on ready made covers, but the last half dozen I made I bought fabric and glue at the fabric store - less expensive but more work involved (have to wrap them up like xmas presents).
I use these traps in the corners as bass traps and at first reflection points as broadband absorbers - they work great.
The mineral wool was $2/square foot, and the fabric was $10/yard. It's about 1/3 yard per trap, so the per unit price was about $20.
If these are stationary, then you'll need something to hang them with. I use standard shelf brackets on the bottom, and some strapping secured to a wall anchor near the middle top to keep them from toppling over. I leave some space between the trap and the wall to improve acoustical coefficients.
If they need to be portable, I'd recommend some framing and stands to make them more durable and flexible with regards to their placement. If I were framing mine, I'd start with a simple picture frame design using 1"x4" glued and spline mitered for strength. I'd secure the traps within the frame by strapping across the corners.