I second that and would get an infared gun if you don’t have.
You also want to push the bedding aside and get the glass temp. There is a good chance the snake will move the beddingor burrow and of you are reading say 89 on the substrate but the glass is say 97 that’s not good.
I was thinking you may may need to get another che. What temps are your room temps usually. Sounds like they may be chilly if the tank is having trouble keeping up. I’d get a lower wattage Che on one side and the one you have on the other. This should solve your ambient temps and get the cool side where it should be and keep the hot side good where it should be.
You just need to tinker and dial it in and that infared will help because where ever you point it you get a reading. So you can move the substrate point and see the glass. Replace the substrate and see that temp. Point in the cool hide and see and so on and so forth.
Just st be careful because the extra Che will also cause Loren humidity suckage
oh and I’d be careful w the “plastic” you are using. Sounds like a menlting fire hazard to me. Most use tin foil or wrap cardboard or poster board or dry erase marker board in tin foil. Plastic of most types can and will melt you don’t want that.