I'm in the process of building a 9" riser. I'm going to be putting 4 berkline's on it.

I have a buttkicker (which I'll be using for the berkline loveseat in front of the riser).

My question is what is the easiest method of mounting the buttkicker to the riser.
I would love to put the unit inside the riser (but it will probably be full of sand).

Anyone have any experience w/ this. I know that you can buy the berkline's w/ the buttkickers in them but I could probably save a few dollars if I could probably mount 1-2 of them on the riser (and utilize my exisiting buttkicker amp).

So, options would be: "undermounting buttkicker to the plywood", or installing multiple buttkickers under (or on the back) ofeach chair (or common connection). Seems to me that you would see the unit this way.

What do you think would be the cleanest way?

You can mount the Buttkickers on top of the platform and put the chairs so the Buttkicker sits inside the opening of the chairs (there is usually an opening large enough to do so, just make sure to check the clearances with the chair reclined AND unreclined). One other thing to keep in mind...doing a Buttkicker on or in a platform assumes it is going to shake the top surface or entire structure of the platform. If the platform is filled with sand and the top is screwed to the platform, your Buttkicker isn't going to do much of anything. You'll need to float the top of the platform for the Buttkicker to work. If you want to mount it inside the platform, you are going to need to fill the inside with insulation, which doesn't add much mass, instead of sand. I'd go with acoustic cotton or mineral wool.
You might also take a look at the Clark Synthesis Platinum and the Earthquake Quake 10.0 shakers. Both are individually more powerful than a single Buttkicker LFE.

check out Crowson Technology

Why would your riser be full of sand and not insulation? your stage yes but not the riser especially if you want tactile response out of it with the butt kickers.
I have Bass shakers in mine and one of my subs ON the riser, works great.
The Bass shakers are mounted vertically to a continuous piece of Hardwood that is screwed to the supports where each theater seat is above it.
http://i204.photobucket.com/albums/bb101/Arwenmark/100_1725.jpg