You're doing many things wrong- sorry.
Many issues here: Never bring home a snake without setting up & testing the temperatures in a tank/enclosure for at least a week* FIRST- otherwise, it's very stressful & scary for a new snake when you have to keep adjusting his home, plus the substandard temperatures + the stress puts their health at risk- it makes them more likely to get sick from whatever they may have been exposed to. (*It takes time for the entire tank + furnishings to warm up.) Hint, you probably need to add UTH (under tank heat) a.s.a.p.
Next question- what is the HEIGHT of this tank? When a light (or any other heat source) is too far away for the heat to reach the bottom, that's not a correct set-up. You should be running a CHE (ceramic heat emitter- heat only, no light) overhead, 24/7- that's much better than a light. BPs & most snakes do NOT like bright light, not even for warmth.
I assume you know that all heat producing sources must be regulated by (preferably) a thermostat, or at least a rheostat ("lamp dimmer"), & you need a way to accurately verify the temperatures. Never just set a thermostat ("t-stat") to the desired temperature & assume that's what the temperature is in the tank where the heat is needed. You need to check & re-check for a while to make sure it's not too hot (excessive heat- over 90* can burn or cause nerve damage in snakes) or too cool (snakes can get sick if they don't have adequate warmth because their immune system doesn't work well when they're too cool- plus, they cannot digest their food unless warm enough).
The cooler your house is (or room where the snake is) the harder it is to keep enough warmth in a tank or enclosure. 68-70* won't cut it for a BP- not even just overnight- the temp. shouldn't fall below 77* at night- Cooler temps. tell a snake it's winter & they refuse to eat-& may also end up fasting for months. Not the right way to keep a BP healthy. You may need to insulate the tank- that can help. Better yet, bump up your home's thermostat, at least until you fix this.
Foil conducts heat, it doesn't keep it in. But to regulate the humidity (you want 55-60% most of the time for BPs, & 65-75% when they're in shed) you'll need to cover MOST of the screen top to maintain adequate humidity. For that you can use foil, plastic, or anything else that air cannot go thru. (not fabric!) I think you've already done that, but thinking it was for warmth.
There is always the chance that the guy you got him from wasn't honest about his set-up. Or he kept his house a LOT warmer? Unless you actually SAW his set-up & verified what he said, I'd take it with a grain of salt.
