Some info on Space Couplers from the makers.
Note that absorption is used above for flutter control.
http://www.resolutionmag.com/pdfs/SW...dsmallroom.pdf
This is all well and good, but the original question did include the words cost-effective. Given that, and the splayed walls, perhaps a simple checkerboard of 703/5 cloud and light angled curved panels would be a good shot here.
DD