Many home theaters have more than one row of seats, with the second and third rows being built upon risers. These risers, for ButtKicker transducer applications, have two purposes; raise and support the seats and occupants, and allow vertical platform and seat movement.
Height for the ButtKicker brand transducer:
ButtKicker brand transducers can be mounted above or below the top surface of the riser. If above, consider people traffic, furniture placement. If under the surface, some simple framing can be done to avoid the extra room needed under the riser. As always, keep in mind any maintenance that may become necessary.
Think "suspension."
In essence, you're constructing a floor above the room's floor. Build the riser similar to a suspended wood joist floor to give the riser its greatest low frequency response.
The major differences between a riser and real floors are the suspension points and tie points (to other external structure). The riser should be free floating (not tied into any adjacent walls), and suspended by short sides only. Use vibration isolators on the short dimension to support the riser from the room floor.
The greatest flex of the riser will be along its long dimension, the riser's short dimension will offer much less low frequency flexibility. Therefore, when possible, avoid any support along the long dimension.