Can you post some pics of the room and the intended speaker and listening positions? Reflections will be everywhere including your HVAC. Will the room be sealed or open to other parts of the house?
Room acoustics is a heavy topic that involves physics. Before you buy or try to build anything, I HIGHLY recommend a little education by reading Dr. Floyd Toole's book. Current wisdom is the treat the bass region (below about 500Hz) with absorption while treating mid/high frequencies with a combination of reflection, diffusion, and absorption. Bass traps that are made from fiberglass should be a minimum of 6" thick with a 6"-8" air space behind it. DIY bass traps should be easy enough to build.
Depending on how 'alive' or 'dead' sounding the room is currently, you'll want to consider diffusion to preserve the upper freq energy. Experiment with listening with the first reflection points on the side walls as bare reflective, diffusive, or absorptive. If you do go with absorption, make it 6" thick as anything thinner acts like a low-pass filter and skews the freq range of reflections striking it and reflections of those reflections. When using diffusion for the sIde wall first reflection points, use something that scatters sound horizontally such as curved polyfusers or QRD's. Polyfusers should be 12" thick and QRD's should be more than 6" deep so that both styles work down to about 500Hz. You can stuff fiberglass into the curved cavity of the polyfuser so that it acts as a bass trap while being hard enough on the side facing into the room to avoid absorbing mid/high frequencies.
Lastly, I'd recommend investing in an acoustical measurement tool to see what your ears are hearing and to compare treatment effectiveness to a baseline measurement to see the benefits or to contrast different treatment locations in the room. Get Toole's book first though!