I vented my bulkheads according to some advice on this bulletin board: by drilling a 1/4" hole in each one, filling them with thickened epoxy, then drilling a very small pinhole in the center of that plug. This allows pressure equalization without letting (much) water in. But I also have hatches, and I made these vents before installing the bulkheads. Given you don't have hatches, maybe you could put in 'inspection ports'. They're small, screw-in hatches that can be found at West Marine -- made by Beckson, I believe, and are maybe $6 or so. They're larger than a corked hole to allow the enclosed area to dry out easier if water gets in.

Dean