It's coming up to summer, so they're really not in need of basking too much. 32 degrees is the standard temps for snakes. By plastic enclosure you mean click-clacks?
Depending on where your thermostat probe is located, you might find you need to set the heat mat a bit higher or lower to ensure that the warm end remains around 32. Use your thermostat and an accurate thermometer and monitor temps over a 24 hour period, adjusting as necessary.
Don't forget to keep an eye on temps in the cool and as well. If they get up above 28ish, you might need more ventilation.