Designed up a new fan which should move more air. The last one didn't seem to move much air at all. Designed on SolidWorks, printed it out, and used some spray adhesive to make a drilling and cutting guide on a piece of stainless.

Getting close.

New fan on left, old fan on right. The new one moves a lot more air.

Also made a cooling box to sit atop the fan motor. I'm using my portable shop vac to suck air through the motor windings to keep it cool. Very Rube Goldberg, but seems to work. Unfortunately, the added air flow from the fan hasn't really helped the problem of a temperature difference between front and rear. I'm still getting around 70 F when at the temp range for solution heat treating. Next modification will be to add a duct inside the oven so the air circulation loop encompasses the whole oven rather than just circulating air near the front of the oven.