Personally I use little cans of wet cat food, lol. But I plan to order some hockey pucks. Others have styrofoam on the bottom of their tanks, which they cut into to route the probe and nestle it in directly under the UTH. That way the probe is secure on the outside of the enclosure but you don't actually have to put it under the UTH. I think Sunnyskies (sp?) has a good tutorial for it.