Here are some simple broadband acoustic panels I made, based on the GIK 242.
The frames are made of 1" thick by 3.5" wide poplar. Poplar is used because of its resistance to warping. Finished panel size is roughly 2' x 4'. The pieces are joined with pocket screws via a Kreg jig, which by the way is a great tool to have! I didn't use any glue with these and they are super strong. The corners are simply butted together because once covered in fabric, they won't be seen. As an added touch, I chamfered the outer edge with a router. This was initially just for looks, but it also helps prevent snags when stretching the fabric over the edges. The absorption material used is 2" thick Owens Corning 703 rigid fiberglass panels, which have well known and documented acoustic properties. I then covered the panels with Guilford of Maine FR701 fabric, which is acoustically transparent and meets fire codes. The fabric is neatly stapled on the back side of the frames. The completed panel gives you 2" of absorber material with a 1.5" air gap behind it, which increases the low frequency performance and gives you a place to hang the panel onto a nail or screw. Performance wise, they should be identical to the popular GIK 242 panel as they use the same materials and specifications. I currently use 4 of these at the first reflection points of the left, center and right front speakers.