If you're serious, search eBay for springs. Ones you want will be about 2-3" diameter and compress to about 2" high under a 115lb load. This will be the dirt cheap solution, yet about a million times better than what you're doing now. If you don't want the speakers raised that much then make a platform and put the springs in the corners, in other words copy Townshend Podiums. This is what I did as proof of concept. Works real good.
Or just buy 4 sets of Nobsound. Four sets will easily support 460lb speakers. Way easier. Of course if you do Podiums that will be the best by far. But even Nobsound will be better than what you're doing now.