Use one of these. It is a printed circuit board with fine circuit traces designed to heat a 9" x 9" heated bed on a 3D printer. It draws about 100 watts and, from experience, heats a box 24" per side to 105 degrees F quite nicely. Box is insulated slightly.
These boards (from this supplier) are excellent and last forever with no smell when heating. Use a 12v computer ATX power supply (many instructions how to convert them to a lab power supply). This board will draw 8 to 10 amps at 12 volts.