Totally cool that you scored a free fridge and are taking on the DIY project!

I'm no expert, but I'd think that you'd want to measure the temperature in the tub, the temperature that the eggs are immediately/regularly exposed to.

I wonder if the insulation being older (that part is assumed by the wood grain exterior and the interior color) impedes the strength of the flexwatt? Maybe you need more flexwatt inside? Trying your herpstat might be the fix, definitely worth trying that first.