I've watched it done, and the strips of cedar can be stapled to temporary plywood 'bulkhead' type supports in lieu of foam forms. Each strip is edge-glued to the next with epoxy resin and stapled in place (staples are removed later) When the entire shape is layed up and hardened, the staples are pulled and the plywood forms are removed. A very light glass mat is saturated with special epoxy (not auto-repair poly) resin and layed over the cedar (inside and out). Another coat of resin or two is applied over the mat, and then wet-sanded down. Then the whole thing is given coats of clear marine-grade polyurethane on the outer surface. It sounds more complicated than it really is. And the end results are quite gorgeous, lightweight, waterproof and incredibly strong. The technique has been around for a long time, and the 'West System' of epoxy resins is considered the gold-standard for this type of work. Epoxy is much better in every respect than the standard auto-repair poly products most of us are familiar with. Anyone with some basic wood-working skills could make some bitchin' bags over the winter season.
Link to the West System Epoxy products: