The most important thing about your cage is that it be secure and that it have the right temperatures.
To accomplish this you need a secure container. I like Sterilite containers with latchable lids ( I also put a weight on the lid for cages larger then baby cages).
You can also use an aquarium .
You then need to provide controlled heat to your cage. You will need a heat source, a thermostat and a thermometer.
Ball pythons prefer under tank heat. Many of these heaters say they will not over heat. That is not true you must use a heat controller with them. Pictured above is an "ALIFE THERMOSTAT" (about $ 30.00) and an indoor/outdoor thermometer and a REPTI THERM heat pad.
In the cage you need substrates ( in this case paper towel, I use this for babies and sick snakes or snakes in isolation and cypress mulch for the rest of my collection), a water bowl and hides( you can use plastic containers and cut a notch in one end so they can get under it). Paper needs to be moistened every couple of days cypress mulch about once a week.
To control the heat in the cage, tape the probe from the thermostat and the thermometer down to the bottom of the container over the heat pad.
Then securely tape the cords of both down so the snakes cannot get under them. Make sure to use good tape that will not come loose.
The heat pad is under the left end of this cage. The probe from the Thermostat and Thermometer are taped inside the container over the heat pad and the Thermometer and Thermostat controller are on the right end outside of the container. I have hides on both ends of the cage. I usually keep the water bowl on the cool end so that if they feel hot they can curl around it to cool down. If you have trouble getting enough humidity in your cage you can also put one over the heat.
Here is the view from the top with the lid. (For larger snakes you may want to put a weight on top in the center of the lid).
The heat pad sits under the container it should not cover more then half of the cage floor 3-4 inches is enough to give a good hot spot.
The thermometer is outside the cage on the end away from the heat so it shows the room temp on the inside temp and the hot spot temp on the outside temp (probe is outside temp). You adjust the hot spot temp by changing the Thermostat (it should be 90 - 95 degrees F. I use the thermometer reading for the actual temperature. I have noticed that the cheap thermostats are less accurate then the thermometers). The room temperature should be around 80 degrees in the day and no lower then 70 degrees at night. To change this you need to change your home heat or use a safe space heater.
Here is the same setup on a baby cage.
You can see the heat pad under the cage on the left. I used an electric soldering iron to melt the air holes in the lid.