I know that many people think 100 Fahrenheit is too high.
But my experience is that, if your vivarium is big enough, this works well.
More important is that there is a place inthere where they can cool down.

In the wild temperatures in the sun easily get 100 degrees F. The snakes takes a sunbath for a few minutes. The shorter the better because when they are warm they can hide themself.

In the vivarium snakes tend to go "baking" for a few minutes at 100 degrees F. and than move a bit to the edge of the heathspot to start sunning at the temp they prefer.

One hundred degrees F. is no must, 68 will do also.