I don't think 45 degree angles and mitering is necessary. To each his own, though.
Nails and glue - spray glue used to hold two 2" pieces of CW 703 together - and a visit to a fabric store to buy cool cheap cotton fabric to match the decor.
Mostly they blend in and don't stand out as they reside in low places up against walls or in corners.
The 703 stuff is really just a few hours of work with a suggested mask with gloves, glue, utility knife, and hammer and nails. Oh, yeah, I painted the wood black and then stressed the edges so the panels look old.
Anything you buy is going to absorb far less sound than the 703 panels and cost far more.
Sound diffusers, however, should be purchased as they are quite cheap (Etsy has some from Ukraine that are cool and really inexpensive) and while only generally a thin piece of wood with foam on the back, they serve the purpose well.
https://www.etsy.com/search?q=sound%20absorption%20panels&ref=auto-1&as_prefix=sound%20ab