Therefore I got now some pieces of birch ply:

Ply wood cutted to order via the net is meanwhile the same price as getting it uncutted from the local DIY market. Doing it that way the construction needs only one single straight 8 mm route.

and some Adam Hall Spray adhesive:

Ready is a moveable acoustic absorber:

Got now three pieces to get started and I hope it will help to improve the room a bit. But anyhow it was fun to do another little basement project.