On my old swing keel portager, I used to just wrap a plastic around it after lowering the keel. Then, after constantly running out of plastic bags, I picked up a rubber stopper that fit the tube and drilled a hole through it and cut a slot, after lowering the keel I'd just slip the stopper on and tap it into the tube.
Planned to take a short length of pipe (bronze from an old plumbing installation) and cutting some neopreme foam to fill it, then mounting that on the tube so it was a no-effort seal. Sold the boat before I did it though.
It doesn't take much to keep the water out, it's normally just splash so anything in the way will keep it out.