Worked out perfectly!
I went through the drain hole in the bottom.
I cut out two pieces of thick insulation foam and used a dremel to carve them so they would fit snug into the drain hole in the interior and on the outside. Drilled a small hole down the middle of each, detached the probe lead from the controller so I could snake it through the hole since the probe end won't fit through the 90degree turn the drain takes.
Pulled the wire through and the plugs are snug and air tight in the holes and now the lead has a nice tight seal.