What is the best material and seaming method for making a waterproof overboard bag?
Technically, the best material for a waterproof overboard bag is one that can utilize “welded” seams. Welded seams are created with special machinery that create permanent bonds by shaking the molecular structure of the two material layers and then forcing them together. The result is a very strong seam that is completely water and air tight. Unless you have the equipment to do this, your only alternative is to purchase a premade dry bag.
You could always make a bag on your own using PVC-coated materials like Shelter-Rite and HH66 Vinyl cement. The HH66 glue will do a very nice job of bonding layers of vinyl to create both air and water tight seams.