Probes should not be used inside a snake enclosure. Unnecessary risk, imo.
Your probe should be secured on the outside of the enclosure and between the heat mat and the glass.
Purchase a temp gun to monitor the hot spot temp on the inside of the tub. With glass, there's a thick barrier between the mat and the inside of the enclosure, so you may need to set your thermostat higher than 90 to actually reach an appropriate temp inside.