I think what I would do is to use blend of Colloidal Silica (CS) and the Micro FIbers at the rate of 2 parts CS to the Micro Fibers and use a fast hardener. You want to make a mix that is about the consistency of mayonnaise. This makes a tacky enough blend that when you press your core into it by hand it should stay put. If you organize your blocks well enough and the entire assembly fits in place tightly it won't sag. When you press the blocks of core into place maintain some pressure for a short bit to make epoxy extrude. Once your core is in place and the epoxy has cured you will get to sand that core some before putting your layers of glass back on. Once you have the core fair you will use the low density filler and trowel it over the core to fill everything including any remaining gaps in the core. Then more sanding...........

Dave Graf