You should have a water bowl heavy enough that it doesn't get tippedI personally like Rickey's idea. It's very creative, although I can't see myself being so outgoing, LOL!
I have my probes on the inside - I don't find anything "wrong" about putting it on the outside, I just didn't hear about doing it that way until after I had mine all set up! But then I do think that the temperature could be a little off, since that is not where the snakes are. I also don't know how heat transfers through glass, so maybe it wouldn't even matgter.
I personally use painter's tape for the probes in both my snake's vives. I initially thought the substrate would maybe get under the tape and cause it to come unstuck, but so far it hasn't and I've been using the tape for a couple of months. To be fair my ball python doesn't dig too far under the substrate because there is a hide directly overtop it where he just hangs out, but my corn has not knocked hers off at all.
I also don't place the water bowls near the probes, just in case something were to happen when I was gone (thermostat malfunction) and the snakes needed a cool place to go, the water bowl is on the other side.
EDIT: After reading about a poster's snake who "floods the vive with urine", I would agree that where you place the probe will depend on the snake!That's a bit concerning though... I've never had a snake defecate so much at once that it didn't get soaked into the substrate...