A waterbath is easily made from a covered Pyrex dish.  Trace the bottom of the beaker on the lid.  Cut a half inch in from the outline and then cut 3/4 inch long notches out from the circle around the hole, at 3/4 inches apart.  A beaker fits in tightly and securely.  Fill the bowl with water so that the water level comes to within a half inch of the lid when a beaker is in place.  With a paper punch, make a hole in the lid for the probe from a digital thermometer attached to the wall at eye level and in an area illuminated by safelight.

I have made a number of them sized to fit the various beakers and jars I use for different recipes.  A nice thing about the Pyrex is that they stack well.  Space is at a premium in my darkroom and studio.