The most important thing is a thermostat to control the UTH. It can over heat and be too hot or burn the snake. Pet stores do not usually sell these. My local petsmart occasionally has them but the quality is not that great. The thermostat is what the UTH plugs into. Then you plug the thermostat into the wall or your power strip. The probe for the therm should be taped with strong tape directly to the UTH under the tank/tub.
You will also need a digital thermometer with a probe to monitor the temperatures. Walmart carries one called a Weather Station. Its in the hardware/paint section by the thermometers and thermostats. Its a small grey unit with a big screen and a probe. Its less than $15.
This probe needs to be placed inside the tank directly on the glass over the UTH. The grey unit then sits in the tank on the cool side. The screen will show you the temp of the probe (hot side) and the temp of the cool side (ambient temp) at the same time.
The 20 gal will be ok if you do it right. The UTH needs to be 1/3 the size of the bottom of the tank. Put it on one side. Pick up 2 or 3 small cheap plastic cereal bowls from walmart or the dollar store. Cut a small hole in each and use these for the hides. They should have 2 identical hides that are small and tight fitting around the snake. Doing it with plastic bowls will save money as they grow so fast the first year and can just throw them away and get the next size up when it out grows the bowl.
Then you can put crumpled up newspaper, fake plants and vines or whatever you want that isn't too high so the BP will not fall off of anything and get hurt.
If you use these things and clutter up a bigger tank with lots of stuff the baby will feel more secure. One hide goes on the hot side, one cool side, maybe one in the middle. Wide and shallow water dish so it helps with humidity and the baby wont tip it over.
You will most likely need to cover the top of the screen if you use the tank instead of a tub. Cover it completely with a towel or use foil or something else and just leave a couple inches in one side for air. They need very little ventilation and you will want to try to keep the humidity at least 60% at all times. More when the snake is going in to shed.
Edit:: As for a thermostat to control the UTH heat, wait til some other members are up tomorrow. Im sure you can get some ideas on websites to buy from and brand names to get for cheap. The only ones I use are rather expensive and I cant think of the names of the lower cost ones right now.