You might have to be careful about putting ridged panels on a flexible boat however. The Mac36 is going to twist as it sails. I think your going to have some troubles. The centre tray could be made fairly wide though. This would cut the tramp size down to about 4 feet on each side and still allow the boat to twist.
Balsa is stiffer than foam.
You may be able to get some Nida-core panels down your way which would be ideal for this use. Nida-core is a plastic honecomb product. They are a little picky about giving out prices but the prices are very good, around 2.50 a sqft if I remember right. All rights reserved.