There are many pourable fillers available nowadays. Here is another incorporating the ceramics,
NIDA-BOND POURABLE TRANSOM COMPOUND is ceramic filled polyester exhibiting exceptional physical properties. In particular, the compressive strength has been documented by an independent testing laboratory to be 3,895 psi (ASTM 695). This is several times that of plywood and 8 to 10 times that of PVC foam. In addition, the failure mode shows elastic yielding before failure. This makes the NIDA-BOND POURABLE TRANSOM COMPOUND material an excellent choice for critical applications such as coring transoms on power boat hulls.
web page
http://www.nida-core.com