Why not use what you have, but better it. 

I created my own warmer by elevating a shop made box (about the size of your water tank) with poly mesh floor (you could put holes in the floor of your tank) and placing a $20 "utility/milk room heater" under it, with the area underneath skirted to force the heat to rise around the calf.  I use this in an unheated barn and get along great with it.  During times of below 0 degree weather, I put a second heater blowing under the warmer.

Works awful good if I do say so, and the price sure beats that of the commercial units.