This morning I sanded back the left half of the bonnet. With a flexible kitchen knife it is easy to cut through the foam. So I worked the shape down until it was roughly where I wanted it then with a light sand it was pretty easy to shape. There are a few gaps and larger air bubbles in the foam so I will need to find some putty or filler on my next trip to Bunnings to smooth out the shape. This evening I had nothing to do so I succumbed once again to art store prices and bought 3 sheets of 5x100x915mm balsa for 24$.
With one of these sheet I completed the remainder of the bonnet and fixed the right skeleton to the left foamed section. I then used the same tape method as before and then used the remainder of the second foam canister to fill what I could of the right half. I also cubed up some of the off-cuts of foam from earlier today so that I could try and get as much filled as I could. Unfortunately it came up a bit short so I will leave this until I have to foam the rest of the top. After setting the bonnet aside to dry I printed out the remaining sections I would need and traced out some of them onto one of the sheets of balsa before cutting it all out.
So far with this project I'm impressed with how light and sturdy the whole structure will be. It still amazes me that balsa wood is so easy to cut with a Stanley knife yet when teamed with the foam it is extremely strong. I am still undecided how I am going to get the bonnet to seal to the rear section and also around the tubing. I am thinking about using a tongue and groove set up for attaching the 2 sections and then having bolts running down through to the foam base.