The sponson tips and cockpit cowling tip on my Sport C were formed in pine lumber. I rough-cut the shape on a bandsaw then screwed and glued them in place. Any soft wood will work here.

The grain of the wood is oriented fore-and-aft.

I did most of the shaping with a spokeshave. A power sander could also be used.

On the Pro Tunnel I used flotation foam, initially poured into crude cardboard forms.

A drawback of foam is shrinkage, noted earlier regarding hull flotation. I don't know how long it takes for flotation foam to fully cure, but it may be best to give it a few days or a week, at least.

NOTE: The Papa Smurf growing from the cowling was not caused by foam sponson tips.

It doesn't take much foam to have enough.

Rough-cutting the tips.

After forming the final shape, I reduced their size further to make room for an outer coating of epoxy.

Epoxy thickened with phenolic micro-balloons, liberally applied. Use a power sander to shape.

A better choice would be a coating of one or more layers of fiberglass, set in epoxy.

Another finishing touch, added to the bow and then rounded over.

I also put wood over the aft ends of the tunnel extensions.