Flexwatt will melt a tub if it is unregulated. Common sense. If a heat source is going to melt any material in or near your snakes enclosure, then it is obviously too hot. If you think a UTH will melt a plastic enclosure then it is obvious that the UTH shouldn't be running just by itself, even in a glass enclosure. Use a thermostat and you are golden.
But do the temps stay the same like 90 or should i lower them down...