I’m just about finished assembling the main panels and ready to move on to the bulkhead. Taping the puzzle joints with unthickened epoxy worked just fine. I assembled the bottom first due to space considerations, and it still used thickened  epoxy for the puzzle joint. It may be due to the thickness of the top panels (only 4mm plywood) that they are filled by the straight epoxy just fine. I had to sand a little bit on Both sides after attaching the two sections of the panels, so it penetrated the thin joints just fine.