I use this in my 3 enclosures. It is $12 at Walmart. I put the unit itself on the cold side and the probe up under my substrate on the hot side. It does humidity also. I have it set to show both Temps and the humidity.

I made a ledge out of short adjustable curtain rods. I have thought about cutting a board to sit on them so it is a more sold surface for my bredli to sit on and not just 5 bars.