My tip and I have it now for 3 snakes and it works amazing is styrofoam insulation under the tank. You can get styrofoam from like a craft store, home depot or lowes for real cheap. I ended up with I think 5 strips that worked perfect. It was like $8 or so and looks just like normal styrofoam.
Just set the enclosure on the styrofoam and that's all you need to do. You can take a razor blade to cut the excess off easy.
Now as far as the temp on the cool side. That might be dependent on what you get the hot side to be. Upper 70s should be a fine target range so long as the hot side doesn't get above 90ish. This is why I recommend a temp gun, you can quickly get readings on all sides and dial it into what you want.
As it was mentioned, offset the RHP by about 3 inches from the hot side wall, drape the probe on the cool side wall and aim for a target probe temp of 76-77. Remember the ambient will be somewhere between that range and so upper 70s is fine for the cool wall. I would start setting the thermostat there (76) and adjust as needed to get the target of 90-92 on the warm side surface and above 75 on the cool side. Might need to play with the temps a bit but give it like 15-20 minutes between adjustments to reach temp.