The original idea for this was something my group did for our research exhibit at IEEE/ACM SC2002. In the right side of the photo with the laptop video wall, you can see our triangular monolith made using duct tape and 1/2" thick 4'x8' sheets of foil-backed foam insulation board. That worked well, but didn't have enough mass to stay put, and applying the duct tape wasn't really a fun job.... Duct tape also wouldn't work well with more robust materials, such as OSB, which also did not look as nice.
The black sound-absorbing panel is available at Home Depot and Lowes (and probably any similar store) is a much nicer-looking and pleasantly heavier material. It's also slightly cheaper than foil-backed foam, at a little under $10 per 1/2" thick 4'x8' sheet. Each monolith requires three sheets. You might be thinking you could cut sheets to make smaller monoliths, and you probably could, but be warned these sheets don't cut easily. When new, the black finish on them also can leave marks on things they rub against... but a day or two of air drying seems to fix that problem. In short, they are nice material, but hard to work with.
The trick is the 3D-printed clips. Three clips hold each edge of the freestanding triangle together, so you need a total of nine clips per monolith. You simply push three clips on the edge of one board and then nudge the mating edge into the clips. Handling the 4'x8' sheets is a two-person job, but it takes just seconds per clip without needing any tools.
The clips need to be strong. The design is shaped to provide that strength with very light filling -- we used 2 shells of PLA with just 8% fill. We also oriented the parts so that 45-degree rectangular fill would brace the key points. It's an easy print. We were in a bit of a rush, so we printed a set of 9 at a time on our MakerGear M2 at 120mm/s with 0.25mm extrusion. No problems at all.
You've probably noticed that the clips have teeth inside to grasp the board better. Actually, the thickness of the 1/2" board varies a lot (over 4mm) and they're all undersized -- the teeth make it easy to push on while allowing each clip to hold despite the thickness variations. Should you happen to get board with a significantly different thickness, be sure to test and scale the clips if needed.