I havnt made one yet but the connections will be soldered, coated in dielectric grease, then heat shrunk individually, then as a pair. So that part will be water proof. Then just unplug it from your battery tender and put the cap back on the tender and carry the extension with you.
If you left it on there you would need some sort of cap for the 4 pin connector to the GPS. Should be easy enough to keep that part dry, i will have dielectric grease on that female end aswell.
I was going to order enough to make a few anyways. I will make some extra and if someone ever needs one I will have them
Im not in a rush to make them. Maybe next weekend I will order the parts and then make them on the next cold saturday.