Dilemma
The inside of the hull was coated with glass fibre resin and chopped matting was added to reinforce the engine mounts.
The bow area of the hull was filled with two part expanding foam that was mixed in a jar before pouring into the front of the hull (it was held vertical to stop it running away from the area that it was needed in).
Once fully expanded, the foam was trimmed to make the final expanded area nice and tidy.
The above model was last seen still running many years after I sold it (due to my move away from Devon) - a long time after it was first built and raced in the off-shore series, so the old style adhesives must have worked well enough to hold it together.
The glass fibre resin was also painted around the inside of the hull (the joints being given extra special attention) to prevent the nitro fuel soaking into the wood as it effectively made a glass fibre "shell" that was easy to wipe-out and keep clean after every running session, so the wood was also protected form getting fuel-soaked.