IMHO the material (hardwood, softwood, plywood, heavy plastic) of the possum board makes little or no discernable difference in sound.
I have one board made of balsawood, and I use a poplar dulcimer box case that I made as another. I've also played on wood, metal, glass and fiberglass/plastic boards and tabletops. A hollowed box, like my upside down poplar case, is much louder than even an ordinary possum board.
I don't believe distance or height above the board makes much difference (unless it's more than an inch above), but any distance is better than a dulcimer set flat on a board or table. A back-to-board gap of 1/4" to 3/8" is just about perfect.
Do not pad (with felt or whatever) the places where the dulcimer touches the possum board or the bits that raise the instrument up. You want a hard connection, not a fuzzy, soft meeting of the two pieces. Fuzzy/soft absorbs sound.