Agree with the comments prior to mine from Fred. If you want to use your riser as a bass trap. You need to design it from the beginning. I tweaked mine a bit to attempt to be a bass trap, not sure of it's effectiveness, but I had the extra materials so, HEY.
Agree with the openings at the wall. Most of the time the riser is positioned in a corner/against a wall where the vents along the wall make sense. In my case, my riser is sitting in the middle of the room and I installed a cleat to stop the couch from sliding off the back if you plopped down on it!
So I put vents on the back sides and front and none on the top. Again, not sure if I wasted my time, but is sounds more solid and does not resonate.
About what to stuff it with...regardless of it's bass-trapping potential, stuffing it with unfaced "pink stuff" will keep it from resonating during you movie enjoyment AND while you are walking on it. Before I stuffed mine it sounded like you were beating an empty cardboard box with a stick. 2 x 12's go with the fluffy stuff. My own personal OPINION, the mineral wool works better in tight spaces (hence me using it in the majority of my build), where there is not a lot of room and you need max absorption. Having 12" worth of space, the fiberglass stuff will be perfect and most likely cheaper.
One suggestion is to put a pad under the carpet that you use on the riser. I skipped that part, in many ways wish I would not have.
The pad under the riser is just fine, and in some ways probably better than just roofing paper (vibration, leveling, etc)