I hope this qualifies for Cheap and Cheerful as I like this circle.
I have been making some (blindingly cheap) room treatments and I thought I would post my corner treatment as I don't think I have seen them made this way before and this may help someone who wouldn't otherwise attempt this. The key is being able to make a frame out of 5 pieces of 1x2. If the frame is a fairly uniform width you can hardly fail.
This one is quite small but I made a larger one like this and the same principles apply. I made this out of Roxul which is common around here (Home Depot carries it in Canada) and is leftover stuff from some insulation work I did at a client's.
I start by cutting a triangle the dimension of the inside of the bass trap and running some plumb lines up the wall from the tips of the triangle. For my larger trap I made the triangle out of mdf and used it as a form to cut out triangles of Roxul. (Also for the larger trap the triangle ends up lying in the corner resting on the baseboard where it is tacked in place. Triangles of Roxul are just stacked on this)

Then I just cut some appropriate widths of Roxul with a 45 on the back. I stuffed the scraps in the corner behind the Roxul. I then stapled temporary retaining straps to hold the Roxul.

Stack to the top.

Make a simple 1x frame. I left mine 1/8 of an inch shorter than the finished height.

Staple fabric to the frame. I planned on fixing the finished frame to the wall with 4 or 6 two inch finishing nails just sunk into the drywall at an angle. If the wood for the panel is fairly straight and dry you won't need much to hold it in place. As luck would have it the total height of the frame plus the thickness of the fabric made for a tight fit, so I just pushed it in place and there it is. Total cost: $5 of fabric (that my spouse paid for

)
