Kaya ng karpintero yan Sir. Basically 2x2 wood lang yan na nakakabit sa wall then sandwiched ng plyboard or plywood. In your case, mas maige malamang plyboard para mas kaya ang BS speakers.
Almost same principle as this (pero dapat may screws/nails para mas matibay and mas maige siguro kung plyboard gamit instead of plywood):
Ganito rin kse ongoing project ko with my surround speakers kaya nasuggest ko sya. Hehe! I was inspired by the floating shelves I saw in Roy's (royal) place. So I thought of using the same principle pero for BS surrounds naman.