I had a eight inch long (1/8" diameter) Dacron, (Nylon will also work)
Cord permanently attached to the tail end of the drain plug. When the plug is installed in the transom drain hole, the cord will "stream" harmlessly behind the transom while underway. Make sure to "touch" the tail end of the cord with a lighted match to "seal" the end so it won't fray and come unraveled.
When the boat is hauled out onto the trailer
at the ramp after a day on the water, I remove the plug and take it up to the trailer winch and tie it to the winch handle where it will stay until the next launch.....where I will SEE the Plug and remind me to install it in the transom drain immediately.
NB