In the field we use brass balls (painted black) to hold our iButton temperature loggers. It's meant to be a modular solution for instances where you already have lots of 1/2" PVC couplers mounted on the shore from other projects, or if you just need a convenient way to quickly swap iButtons during low tide.
The round brass ball provides a constant projected surface area facing the sun for almost any direction, which means you can rely on having consistent, standardized exposure as the sun moves through the sky. We ordered our balls from Liberty Brass. They are willing to drill and put threads in the balls (if you order enough to make it worth their while) so that you can easily mount the balls on 1/2" PVC fittings.
The appropriate size to order is the 1.25" ball, but with a special order 1/2" NPT Pipe thread tapped into it. This allows room for the iButton to fit inside the ball, and allows you to screw in standard PVC pipe fittings. They don't seal completely watertight, but the older iButtons tend to hold up fairly well even when submerged in salt water.
The balls are painted black with an epoxy paint so they soak up the sun's rays in a predictable fashion. The simplest way to mount the balls on the shore is to use 1/2" PVC pipe plugs. We drill a hole through the center of the cap, and put a #10 lag screw through the hole into a standard blue rock anchor. It is also important to put a layer of Z-spar epoxy around the base of the pipe plug (after you've scraped the rock clean) to help hold the pipe plug firmly and keep it from turning when you're trying to install or remove the brass ball (see the top picture).
We place the iButton inside the ball, and put a piece of foam in as well to help press the iButton against the inside surface of the ball when the ball is screwed onto the mount in the field. A pair of rubber-coated gardening gloves is handy for getting a proper grip on the brass ball when installing it in the field.