[Anthony]

I can't say what the best is (probably the free floating on rubber isolation mounts), but here's what I did:

Ladder frame of 2x8's on the floor. Floating, not fixed to the wall (it touches three walls, so movement is not a problem).

Ran conduit underneath and stuffed with insulation.

Two layers of offset plywood (no overlapping seams), different thicknesses. I did not glue them, since I have needed to get in there to run more wires on occasion. I did use a ton of screws, though. Leave about a 1/2" to 3/4" lip over the edge to make a nice carpet transistion. You could do more, but I was afraid of the plywood breaking with too much overhang unsupported.

Carpet the whole thing, including the vertical faces. I hid poor transition points (carpet to wall and vertical to floor) with molding.

It's solid and doesn't suck the bass out of the room, or resonate.

Be sure to draw plenty of diagrams for elevation to make sure your viewing angles are okay.

So far that's all I have. There's an article somewhere on the internet where someone did their riser as a Helmholtz resonator to help with the bass absorption. If I find it, I'll link here.

Good luck.