I'm glad I didn't go with the room within a room approach. I'm currently trying to build broadband absorbers, but getting Pete Corning's fiberglass has proven to be very complicated in Germany. Although they seem to have distribution here, their contact information is nowhere to be found.
My second option (by the Austrian company Knauf insulation) DP-5 (50 kg/m3 - 3 lb/ft3 rigid rockwool boards) or similar are also incredibly difficult to find here, and haven't been able to get their acoustic absorption data sheets. I'll post the info here if I manage to find something useful.
Thank you very much for your reply and for all the guidance you offer to your fans and followers. I wish you all the best with your book, and I'm really looking forward to buying it!