Seek a copy of the building instructions from the national web site. http://www.theracingcat.com/index.php?page=the-shop Decks can be 4mm gaboon but it must be supported. Several options; stringers running forward/aft or I beams across especially for the mid decks. This latter method is very strong, light but slower to build. See the instructions for both methods. No need for glass, this just adds weight, but epoxy resin (2 coats) must be applied to undersides before attaching ply to the hulls.
PM me if you need specific details as I am very familiar with timber build.
Reread of post appears hulls are glass/foam sandwich. Nothing stopping you from using timber build methods to close in the decks. I cannot source 3mm gaboon ply anywhere in Oz. If anyone can, please tell me where!! As before 1 layer of 4mm will hold up (17+years) if engineered properly. (I have seen some failures due to poor/quick build methods.) Oregon for beam pads will be strong. Source some older samples with close growth rings, (no second growth as the tree grows too quickly resulting in inferior quality). Fix pad across the hulls with the annual rings horizontal so the bolting force is across them. Using large bearing plates under the nuts will spread the loads so the timber will not be compressed. I used 2 -3mm aluminium. All timber to be coated with epoxy resin, as in the WEST system. See the building instructions.