Another way to achieve a waterproof connection is to avoid the exterior plug
altogether. Instead, mount a Blue Seas Cable Clam (preferably in a winch
handle compartment or other spot out of the weather), install the power cable so that it screws into a buss bar or heavy terminal strip located below decks. It's not like you plug and unplug your panels
on a daily basis. Instead, you undo two screw terminals to disconnect the power wires, then unscrew three collar screws on the cable clam. These things are considerably more dependable than O-ring seal and threaded connectors, and you can use any size cable you could wish for, up to 4-0. The newest ones even have stainless caps to protect against UV decay.