This is the most convenient thing you can do as per the condition. You should go for it. Make use of the metal rods between the spaces below. It will strengthen yopur floor
Actually these local fitters don't really does the work good. But your problem about the cavities can be solved. You can use slightly bigger glass film I had recently replaced glass film with the film nowadays coming with impact windows miami or you can use a sort of liquid that can seal the...