Coupling nuts are typically just threaded all the way through with right handed threads. They are meant to connect two standard threads, not to be used like a turnbuckle.
There is room inside the handle to easily accomodate a simple lead ingot 11/16" thick, 1-1/4" wide, and 2" long, which would weigh almost 3/4 of a pound. But anything that you could fit into the handle and drill with a 1/4" hole could be used as weight. Likewise, any wood or closed cell foam could be shaped into a plug to add some buoyancy.