Hokie, will you mail me a magnifying glass so I can see your pics.
If you do mount them inside the riser, make sure you design an access panel that will allow removal/servicing should the need arise. I originally had mine mounted in the riser but couldn't get rid of a resonation that they were creating so I pulled them out.
Edit: I was thinking you were using buttkickers. I don't think bass shakers will have near enough power to do what you need if mounted inside the riser, IMO.