I would be hesitant on the balsa idea due to rot. Inevitably the sole boards will see water migrating through to the core. These look to be fairly well supported panels (limited spans) wither the accept ion of the bilge board. If replacement of just the bilge board solves the issue, try http://www.boulterplywood.com/ for 1/2 & 1/4 sheets. If you want to go for a laminate I would look at foam core and glass, probably a layer or two of 1708 top and bottom with paint/skid to preference. Gucci for these goes to carbon and wood veneer. Regardless of what material level you decide on think about adding some uni fore/aft capped with a layer of woven on the backside of the bilge board. While the cost of the teak & holly sole is not small, it will be less than high quality custom composites. Maybe look at a thinner plywood with carbon uni on the bottom for the bilge board. Good luck.