Question on Ceramic Hea Emitters

[Luthien]

Ok so as most of you know I'm trying to decide on decent priced way to heat my renevated wood vivarium for my corn snake. I've played with the idea of heat cable (just about ruled that out), heat panels (to expensive) and now I'm thinking of the CHE.

So here is my questions on the CHE:
1. Can these "Heaters" sit on a screen or do they have to be suspended above the cage?

2. If my viv is approximately a 65 gal what wattage would I need to heat 1/4-1/2 of it?

3. For those of you that use/have used these do you leave them on 24/7 or do you turn them off at night? Is it safe for the corn to not have heat all night?

Thanks!