I built one years ago. Used a 48" X 48" sheet of 1/4" plexi. On each side of it is a 1" hardwood dowel, fasted to the plexiglass with aluminum angled brackets (like angle-iron). At the base of the two dowels are "feet" made of pieces of "1X"3 about six inches long. I painted all of the wood pieces flat black.
This "shield" works great at deflecting the in-your-face volume of guitar cabs. But I would caution that placement is key. Gotta keep the shield at least 2 feet or more away from the cab or you get this weird tubby thing going-on with those sound waves bouncing back on each other. I rarely use it anymore. Actually... I keep it in the doorway to my man cave to keep the cats out (so they don't use my cabs as scratching posts), but so I can leave the door open.
Like many neat ideas, the practical usage isn't as "be-all-end-all" as you'd think....