Check the larger hardware stores in your area. If you don't have access to sheet plastic, try cutting an XL Rubbermaid lid with a sharp razor to fit the bottom. Put the flex watt heating underneath and secure the plastic to the cage floor with silicone or some other plastic safe sealant(no sealant near the flex watt of course). You want to have a tight sandwich between all layers for good heat transfer. Unless you can find very low wattage flexwatt you will likely need to use this set-up in conjunction with a thermostat.
I hope this helps. He should be fine with the bulb in the short run...but for the long run, low watt flouro, heat tape, and a thermostat would be ideal for the python.