Temps and stuff

I have a red tail boa that is still small ( 24" at most). He is in a 20 gallon tank atm. I have a UTH with a temp controller that is set at 90. The bottom of the tank is reading at 87. My boa spends most of his time on the hot side. The cold side is around 75. Would a ceramic heater bulb be good to get the ambient temp up? Would replacing the screen top with a cut plexiglas lid be better for the humidity? The humidity stays around 60 -70% but I want to keep it up around 70%. He is not super active( compared to my corn snake) but does move around. I'm hoping if I can get his setup perfect he will be more active.

He is kept on aspin bedding, has 2 hides, a large climbing stick, and a large water bowl that he can get into if he wants. I will post a pic of his tank later when I can. Thanks

I don't keep boas, but figured I'd chime in with general tips to help with temps and humidity.

Covering the screen lid goes a long way to help with heat and humidity.
Adding a CHE will definitely help your ambient and cool side temps.
Keep in mind, the CHE will also drain humidity, so covering the screen lid will help that.
Aspen will dry out real quick too, so you might want to try a different substrate. I use a blend of eco earth and repti-bark. It's great for humidity, odor control, looks natural and is easy to spot clean.