Cut the interconnect so that you have two rings with three threads (or one with four and one with two). Debur/file everything. Thread one of the rings in all the way in, then place the lamp in so that the ring holds it and thread the other piece in, verify that they are the correct size.
One of the filed rings inside the housing.
You can put a gasket in if you'd like, to make it more waterproof.
To cap the other end, find a plastic jug cap that is 1.5" diameter. These are surprisingly common, and I got mine off of a thrown out motor oil container. Cut the threads out with a dremel or knife. Drill a hole and mount the potentiometer. Drill another hole to let the two battery connect wires through.