Vibration Isolation for Turntables

Would love some tips for isolating vibrations in a fairly strange setup in advance of an event.

From top to bottom:

• Subs on the ground in the center of the stage
• The stage is made of plywood and built on top of the subs (the entire stage is a bass trap)
• The booth is made of plywood and is elevated on casters
• The turntables are in coffin-cases on spring isolation feet (these solved turntable feedback issues from the subs, but not skipping)

When people are dancing close to the stage and the dj moves around, records skip.

It's definitely a mass issue, and I'm wondering if y'all had ideas about where we can maximize adding mass to isolate the booth from the stage. Do we throw a ton of cinderblocks into the booth? What do we do about the casters, which transfer energy through a small surface area?

Thanks in advance.