Skip the plunger. They don't seal well and are prone to falling over. Unless you're keeping a close eye on things, you could run dry and not know it.
Bucket fed by a garden hose and a 1.25" flexible PVC hose to the transmission cooler or the water pump (whichever is first) takes all of a minute to get set up and is infinitely reliable.
Previous: 1993 Prostar 205
Red 1998 Ski Nautique, PCM GT40, 310 hp, , Acme 4 blade, Perfect Pass SG/Zbox.
Be kind. Have fun.