Why can't you use a seed mat like a heating pad? Just prop it up 2-3 inches and cover it with something, then check the temperature under the mat. Though I have never used a heat mat and just assumed they let off about 90 degrees.
Heating pads are cheap, but another handy device is a dimmer cord (under $10 at hardware stores). Dimmer cords let you control the power/heat output on bulbs, mats, etc...
Those super hot heat lamps are dangerous especially in a coop, plus the light keeps the chicks half awake all night long. When I occasionally add some heat to the coop (if the temp drops into the 20s) I use a 74 or 100W ceramic heat emitter bulb for reptiles