[mcguyver]

Plate carriers are designed to fit with plates in them. I used those Toy Soldier plastic plates (from 204 Tactical) in both the Wasatch and a CIRAS. Those rigs fit way better with the plates in than without.

Try sterilized play sand (available at any home renovation store). It will add mass, and still be removable if needed. Get a hole filler from Crappy Tire (usually in the automotive section, the plastic ones used to fill grommet holes in cars). Drill the correct hole size, fill with sand, then plug the hole. Make the hole on the top of the plate, not the bottom, in case the filler ever pops out.