Absolutely tomic601. I designed and built my own 4 cu.ft. (internal) enclosures for the Rythmik 15" sub kit, and incorporated Danny Richie’s idea of a "sandbox" (it is viewable on the GR Research website). It is an enclosure within an enclosure, the 1/2" space between them (supported by "ribs") filled with sand. Deader (non-resonant) than a doornail, and very heavy. The inner box has a cross-brace every 6" in all three planes (top-to-bottom, side-to-side, and front-to-back), and the baffle is three layers of MDF (2-1/4" thick). I drew up the plans, and had a local cabinet maker cut the MDF into a flat pack, which I then assembled (glue and clamps) and painted.
I also made some constrained-layer platforms for the tops of my Solid Steel equipment stands, putting a layer of ASC Wall Damp (similar to E-A-R Isodamp) between two of the top shelves. While Maple boards are popular for their pleasing "musical" sound, I prefer no sound from support platforms, same as speaker enclosures.