The thickness of the laminate will affect its stiffness. Stiffness can be increased by adding structural supports to the backside of the laminate, such as bulkheads and stringers in a boat. Another way to increase stiffness while minimizing the weight of carrying a solid glass and resin cross section is to use a core material.
Core materials are fairly wide ranging. They include wood; especially end-grain balsa and plywood.
Plastics such as foamed PVC, foamed polyurethane, honeycombed polypropylene, and several others can be used with success. Paper honeycomb and cardboard can also be used with success in the proper application.
The biggest key to successful core use is successful adhesion and capture of the coring material. It needs to be part of the laminate in order to be beneficial. Each material has their own downfall, and these must be considered for each application. Wood and paper rot if exposed to water. Some of the thermoplastics materials melt and deform under high temperatures. Some are too expensive. Plywood and Balsa have density ranges across the sheets, while plastics are much more controlled and consistent.
Ideally the core material is placed directly in the center of the cross-section of the laminate so that the neutral axis passes in the center of the core material. This balances the loading forces of compression and tension under loading conditions from either side. The thickness of the coring can be determined through laboratory testing of panels differing by only the one variable of thickness.