I used super soundproofing (3/4" thick) from ACS:
http://www.aircraftspruce.com/menus/cs/sound.html
and I bent a couple of pieces of 0.040 Aluminum sheet to use as floorboards. They slide under the longerons on the sides and have a bent 3/4" flange that mates to the tunnel (the bend is down and there are a few holes in it to accommodate the screw heads for the tunnel screws - that tends to hold it in place on the tunnel side).
I put a couple of pieces of skateboard tread (very cheap at your local skateboard aircraft store) to keep from skidding my feet around on the aluminum.
Greg