Mike,
It turned out I found no need to actually bond the steel sheet to the base. It seemed simply placing it between the maple block and the MDF - no glue or damping to bond - worked fine. Whatever type of wood I sat on to the steel sheet, the combination immediately became more solid - the wood having a slight ringing "knock, knock" tone when rapped without the steel, and a very dead, dense "thunk, thunk" sound when placed on the steel.
That was the isolation base though. When I added the other steel sheet to the very thin MDF top shelf on my lovan rack, in that case bonding it (with all damp) made more sense, because the MDF shelf was so thin it could slide a bit over the steel if the turntable above it happened to be pushed. But with the steel sheet in the turntable isolation base, just the natural weight of the wood placed above it (and the turntable) provided tons of friction and it's really solid.