My advice, just turn it off until you can get the rheostat. Sometimes putting more stuff on top of an unregulated heater will actually make the surface even hotter should the snake come in contact with it.

The snake will be okay for a few days at room temperature. Just don't feed the snake until you have the temps under control.

If you can afford it, go ahead and get the thermostat instead of the dimmer and you won't have to adjust it constantly.