" Have you thought of using something like this? http://tinyurl.com/3y8z55
You'd have to make sure you had good ventilation, of course.

With regard to frost protection, the formula is exactly the same as that
used for calculating normal heating requirements, but the numbers are
different. Decide what temperature you want to maintain under what
conditions - e.g. 5 degC inside when it's -5 degC outside (say) - then
work out the heat losses, taking into account the U value and area of each
external surface (walls, windows, doors, roof, floor). Whatever heater you
put in there needs to be able to replace the heat which is going out. A
greenhouse heater of a few hundres watts may well suffice, but you'll need
to work it out.
--
Cheers,
Roger

Radiant heat heaters heat YOU if pointed at you - not too close though, else
you may combust !!

Little point in trying to maintain the whole room above ambient - it would
take a load more heat input to do that
without good insulation.

Nick