I use a UTH attatched to my tub for mine (and a UTH attatched to my 20L for the corn as well with no cracking to the glass): I don't use CHEs (heat lamps) on my tubs as well as the tank and no one needs them. If you don't have enough for a tstat pick up a dimmer for $10 bucks at your Home Depot, this will ensure that the UTH won't possibly burn your snake in the meantime of getting a tstat. Along with the dimmer since they don't regulate the temps to a certain temp that you set it at (it fluctuates with the ambiant temps that the room is at while a tstat maintains the temperature to the temp that you set it at regardless of the room ambient temps) I would pair it with a thermometer with a probe set above the UTH under the substrate and/or have a good temperature gun at hand to check the temps daily - I picked up mine from Lowes for $30.

For the smell of the ferret making the King hungry, antsy, or aggressive: I wouldn't worry abut it as this is, IMO, very unlikley.