The bass pans require a different setup for sinking. I still use the Lower Base and the Lazy Susan with the casters.
However, the bass pan is mounted to a wood and concrete "sandwhich" and held down with ratchet straps.
The sandwhich is made up of two circles of 3/4" plywood, separated by 2x4's laid on their side. The circles have a diameter equal to the outside diameter of the pan, plus an allowance of 2 inches all the way around. Typically the OD of the pan is about 23", so the plywood circles are about 23+2+2=27" in diameter.
The 2x4's are arranged in a hexagonal fashion around the outside of the circles and are screwed down. The space inside the 2x4's is filled with concrete. Four large metal stainless steel lugs are mounted to the edge of the disk to provide a place for the ratchet straps to hook onto. The disk has a hole in the center that the center post runs through. Around the outside of the upper circle I screwed some 3/4" plywood arcs that act as positional holders for the barrel to stop it sliding off the plywood.
The ratchet straps have large metal hooks on the ends. The hooks are longer than the lip of the barrel, and so will dig into the note surfaces when they're tightened down. To solve this problem, I made four small metal s-hooks that fit over the lip of the barrel and to which the ratchet straps occurs.
Of course, this entire assembly is quite tall...in the photo you'll notice the box on which I stand when I'm working on these.