I wouldn't worry too much about the air bubble, especially if it is small. The heat will transfer fine especially on a plastic tub.

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Not knowing your setup all I will say is please make sure that your heat source is attached to a thermostat or rheostat as heat pads and heat tape can reach temperatures of 120F which can result in serious burns to your snake. Hook up a digital thermometer with a probe and don't be using any tape to fasten the probe if it is inside the tub.