Are you looking for that old Delta Blues sound? Why not use the same slide that all the old time musicians used back in the day? A piece of copper pipe is the way to go. These slides have been cut, smoothed, and polished. Just slip it on and you're ready to go.
These slides are roughly 2 inches long and have an inside diameter of roughly 3/4 inch. It fits well over medium-sized fingers.
Please note that copper is a fairly soft metal, so the slide will get scuffed and scratched by the guitar strings as you play. If that bothers you, you can get rid of the scratches and polish it back up with fine sandpaper, steel wool, or a buffing wheel.