I've been toying with this idea lately but I thought I'd let you guys comment (destroy) on it. Haha kiddin. I need to here from you if there's an obvious reason why this wouldn't work.
The hydrometer would be inserted at the top of the PVC tee that would have a threaded cap. The bottom PVC tee also would have a cap that could be removed to assist in cleaning. The clear section of PVC would be Harvel ClearPVC.
One option i've also considered it a hand pump primer button on the bottom of the bottom tee that could assist in circulating new wort in to the sight tube.
I started this with google sketch, but I suck at it, so I just finished it with a paint program.