Warming up metal plate safely
Figure 1. A 20 W wire-wound resistor.

The simplest thing would be to just heat the plates using a resistor mounted on the back. You could get a hunch of how much power you will need for this by carefully placing your hand on a 20 W lamp to feel the amount of heat it gives out. That should give you a rough guide to how much power you need per plate. Remember that your plate will be dissipating the power over a wider area so the temperature will be lower.


For safety you will need a low voltage supply. Let's say you go for 12 V which are common.
Next from \$ P = \frac {V^2}{R} \$ we can calculate the resistance. If we choose a 20 W resistor it will get very hot at the rated power so we should run it at 10 W to make it safe to touch. So rearranging the formula you can calculate \$ R = \frac {V^2}{P} = \frac {12^2}{10} = 14.4 \ \Omega \$. 15 Ω is the nearest standard value.


One or two of those wired in parallel on each plate should do the job.

Connect all your heaters in parallel.

Total current will be \$ I = \frac {V}{R} \cdot n \$ where n is the number of resistors.