Even in an open rack, heat rises, and the top will be warmer than the bottom and that's why people put the probe in the middle. I would suggest adding some adhesive backed weatherstripping to the front edge of the shelfs so the door seals up and doesn't allow hot air transfer in the front. What you might have to do is put more holes in the plexi-glass front at the top shelf and taper the number of holes downwards.
The back heat tape should be consistant top to bottom and the consistant venting there should keep hot spots equal, but ambient temps are going to increase from bottom to top.