I bought the NIX at Walgreens, and then changed my mind and tried finding the frontline spray. I went to three different petco's and finally found it (and it was on clearance for $12).
It was quite the hassle rubbing them all down and tearing everything out of the enclosure. And I decided to treat my new male ETB while I was at it, and it bit me right on the finger.
But I got it done. And I think it'll be most effective to treat the snakes directly, instead of spraying the enclosure. But we'll see. Hopefully there's no adverse effects.