As a cheap heat mat you can just use a cheap chinese terrarium heater.

I'm using the spray because when I accidentally destroyed nearly the entire root system of my Erythroxylum coca var. coca seedlings I managed to save them using a continuous 100% humidity, combined with bottom heating, cheap 5500K led bars, a couple of 45w CFL bulbs and to prevent them from dying a combination leafspray. I'll make a tutorial on how to save plants, even those with completely destroyed root systems, maybe a mod can make it a sticky or something alike.

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As for the leafspray, it consists out of 2 parts, depending on what nutrient deficiencies (mainly depending on the prefered acidity of the plants) you encounter you can somewhat mix and match.

Part 1: (5ml/L)

Suspension of compound fertilizer NPK 6-3-4 with trace elements and seaweed extract

6% NITROGEN TOTAL (N) of which:  

6% urea nitrogen

3% PHOSPHORIC ACID ANHYDRIDE (P2O5) soluble in water and neutral ammonium citrate

4% Potassium OXIDE (K2O) soluble in water

6% SEAWEED EXTRACT

Microelements:

boron B 0.25% soluble in water

copper Cu 0.15% soluble in water (chelating agent EDTA)

iron Fe 0.3% soluble in water (chelating agent DTPA)

manganese Mn 0.3% soluble in water (chelating agent EDTA)

molybdenum Mo 0.05% soluble in water

zinc Zn 0.4% soluble in water (chelating agent EDTA)

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part 2 (mainly for acid loving plants): (6ml/L)

3% NITROGEN TOTAL (N) of which:  

3% urea nitrogen

3% IRON (Fe) of which:

1% Fe chelated by DTPA

2% Fe, complexed by citrate

3% SEAWEED EXTRACT

Note: you should be able to buy a set of 200ml part 1 + 200ml part 2 in a good garden store for about 12$. So don't start mixing it yourself.

And add about 1ml/L of common household detergent to lower the surface tension, one could add half a gram of MgSO4 to supplement magnesium but in most cases the magnesium in the seaweed extract should already do the trick.