A tompion served the same purpose a tampon ( derived from the same root word in french) serves. They keep fluids from leaking. A tompion was usually a wooden plug, with some kind of leather, or wool felt liner that fit the muzzle of the barrel closely, and kept rain out of the barrel when the guns were stacked during the night in front of the tents. In order to be able to identify his gun at night, each soldier carved the head of his tompion his own way, so he could find it by feel. I have one made for my fowler, and its basically a wooden clothes pin, except instead of fitting over a rope line, the split in the stick allows the stick to be steamed and opened, so that there is spring in the stick when its shoved down the muzzle to hold the tompion in the barrel firmly. That way, the tompion can be left in the barrel during marches in the rain, with the gun on a sling muzzle up. The tompion keeps the rail out of the barrel. It didn't do anything about moisture getting through the vent, or water getting into the pan, but at least the barrel avoilded being rusted. My tompion is about 5 inches long, with 3 1/2 inches being the part that sticks down into the barrel. I saw cut makes to or four " wings" and I soaked them in boiling water for more than an hour, and then used knife blades to spread them out while they dried. Seal the wood with a good stock finish oil, and then use a good oil in them to help them slide into the muzzle. YOu do have to pinch the ends together to get the ends down the barrel, or at least started. Then the outward pressure of the ends holds the tompion in the barrel. The knob that sticks out of the barrel has a leather, or felt washer to help seal the muzzle, glued to the underside of the knob. Since I don't know anyone else locally who uses a tompion, I have not felt a need to carve this one, yet.
I would not use a Tompion in the field to keep rain out of the barrel. They don't fit tight enough. Use plastic wrap and rubber bands, a condom, as was used by allied forces on D-Day in 1944, or a good tape. Electrician's tape does not stick very well when it gets 45 degrees or colder. At freezing, it often just falls off. I think duct tape will stick better. The air in front of the PRB will blow a hole in the tape, or blow it off the muzzle long before the ball reaches it and leaves the barrel.