Small heating pads or belts can be placed directly onto the vat. Large heating pads can be placed on the inside or outside of the printer cover. These can be secured by nylon rope, rubber bands, or a temperature resistant adhesive. Do not use anything that could puncture the heating pad. Small heat pads are efficient since they are in direct contact to the vat. The main downside is that they have no straightforward method of temperature control, which results in an inconsistent resin temperature. Also, you would need multiple heating pads, one for each printer and bottle of resin to preheat.