Buying an advanced care snake without the money to properly care for it was your first mistake.
Trying to use a 40 gallon breeder is the second. They don't hold humidity well and is to small for a adult BRB anyway.
Third if the uth you are using is not controlled by a thermostat, UNPLUG IT NOW. Uncontrolled UTHs can really hurt and even kill your snake. BRBs do not need high temp hotspots and the uth is guaranteed to be too hot for him. And if you covered it with substrate and think it's safe, you are wrong. Your snake can and will burrow at some point. Just unplug it until you can use it correctly. Google burned snakes to see what can happen. If you don't have money for a tutelage you for sure don't have money for an emergency vet visit.
As to rigging this tank it can be done. Humidity at 75% is good, if that is what it really is.
I suggest two things.
1. Get a large bowl the snake can soak in. My 6ft boy loves soaking no matter if he is in shed or not.
2. Get frog moss from the pet store. Make a big clump damp and place it in the hides. This will create higher humidity in the hides. This will not raise the tank humidity but will keep it high in the hides and since that is where the snake will be, it is benefited greatly by it.
You may check around Craigslist for a used PVC cage. You would be much better off with one.
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