For those who don’t mind getting their hands dirty:
GR Research has just introduced DIY acoustic panel kits sold as flat packs. Just screw the Baltic Birch pieces together, stuff the assembled frame(s) with your material of choice, and cover with fabric. Sizes and priced on the GRR website, a video about the kits on YouTube (just do a search for the company name, the video will appear). The kits are far cheaper than factory-assembled ones, as is shipping.