The graphic isn't running on my machine here.
But anyway........ How deep is the indentation?
If you want to build gobos, this is a good starting point.
http://johnlsayers.com/HR/index1.htm
Here's a pair of folding panel gobos I made two years ago. I'll try to post a pic of my current setup later tonight.
http://www.homerecording.com/bbs/showthread.php?t=99265&highlight=c7s+gobos
The gobos look great, and they provide some good treatment around my listening area at this time. One of these days I'm gonna build a set of John's corner slot resonators. I have a 60" x 40" x 3-1/2 << 5-1/2" deep slot resonator in one of the panels. It was a cool experiment and it works. It's tuned to center at 250Hz, and the broadband is from about 110Hz to a bit over 500Hz.
If you wanna go on a little tangent, check out the studio pics at John's site. Blue Bear has slot resonators in his rooms in order to give them a different "sound" i.e. brighter/darker, and I think he has pics of a drum room up there too. It's very simple technology, easily adapted to gobos. As a bonus, the varied height of the slats gives you diffusion as well.
A search around here for "sound treatment" or "slot resonator" should yield some results as well.