This is a much covered area of the recording environment build process, how to make broad band panels. And one of the best places to get accurate information is still www.johnlsayers.com. You may have special requirements that a general type absorption panel will not cover, to that I offer this linked page of many types and designs of wall mounted panels to better meet your needs.
Hope something fits!
How to: Acoustic panels
Corner bass trapping model

How to make a Bass Trap Acoustic Panel (Tutorial)