The standard deck connectors found in chandlers are rubbish and only last a year or two. The only ones I have found at a reasonable size and price that work are made by a German company called Binder...

Binder Connectors

Series 692 are suitable. The problem is then finding a retailer that will sell you a small number. Compass Watersports have a measly small selection, for example...

Binder connector at Compass

Using these with a yearly vaseline application, and a folded-over full length rubber boot over the whole lot, I've got nearly 10 years out of them now with no problems. The deck of a Dragonfly might not see oceans, but it is a pretty wet place all the same.