After some research and suggestions from various forums, I decided to use an aerosol foam to fill the gap between the hull and new floor. It seems that the conventional method here is to use epoxy mixed with a filler (ie microfibers) to fill the gap and seal the floor.

I'm satisfied with the floor bonding, and with the fit of the pieces, so I decided to try something cheaper / easier to work with.

This foam is really cheap, available at your local Home Depot, and has both adhesive and support properties. It's easily trimmed / shaped for a perfect transition, and cures quickly.

I'm not sure how well this will last, but it seems like a good idea, and worth a try.