Richard,
First, I apologize that I am going to post and run, have to get to work.
Please do try it outside. I am not intending to be an alarmist, but I have concerns about what you are trying. In my setup I had a 100 watt heater in 5 gallons (20 watts / gal) with plenty of water space around the heater. You will have 150 watts per gallon and less water area. Usually a heater will fail and turn off. But if it fails and stays on, there is more of a risk of overheating and with closer proximity of plastic to heat source, more risk of melting and worse. The plastic of the jug shown is also very thin. If you decide to go further than this experiment you might water to consider a new unused gas can for the thicker plastic. I would recommend that you use duct tape at the joint between the heater and the jug and leave some expansion space for the water. You don't need any evaporation.
I'll be back late this afternoon.
