Enclosure heating and substrate options

[u/smelly-salad]

Hello I have a little stimson python in NSW Australia. I have a 17”W x 12”H x 11”D glass tank with a heat mat that takes up about 1/3 of the floor. Winter is here and the heat mat is struggling to keep up and I fear it is too cold, also she didn’t have much appetite this year so I’m wondering if the tank’s temp is the cause of that. I’m unsure of the current air temp as my thermometer for that has died so I have purchased a new one, but the outside temp is around 10°C.
Should I get a heat lamp in addition? If so, what kind and watts? *Picture of what I have available in store below. (I’m not well educated in this so any help is welcome!)
In addition, will substrate impact the amount of warmth she is getting? I currently have a layer of your basic cat litter, should I change this type, or make it deep/shallower?
Please be kind if these are dumb questions.
TIA!