Suggestions for speaker stand platforms?

My speaker stands are just barely tall enough and I'm considering putting them on platforms for extra height and to lower vibrations being transferred to the building.

My monitors are mounted on heavy stands spiked to the floor through a thick carpet and pad. My listening seat semi-reclines theater style. In the reclined position the speakers are just barely at the recommended height relative to my ears; the speakers are too low when sitting up. My living room is in a condo above a garage, but I'm still concerned about resonance bothering my neighbors.

My thought is to put platforms of a dense material under the speaker stands and down-firing subwoofer. The main goal is to elevate the speakers 2-3" while keeping the stands coupled to a massive object. A desirable side effect would be to reduce resonance being transferred to the building. The sub would probably benefit from firing into something more solid than the carpet, too.

Is this a reasonable solution or are there holes in my logic?
What material for the platforms would be best?
Should the platforms be bolted to the stands or should the stands be on spikes?

If more information is needed please feel free to ask!

Thanks in advance for sharing your insight and experiences.