Warping when printing with ABS

[tmostad 5]

I have not been able to successfully print a large item with ABS due to warpage at the back left corner. I have been trying to print a chassis for a project I built. I pushes the limit of the available width and depth of the UM3E at 8 inches long. I tried rotating the part and printing it both ways and each time the back left corner lifts off the bed. I am using MatterHackers Pro ABS filament with Wolfbite ABS solution applied to the bed. I followed their recipe for speed, bed temp, etc. On a hunch I used my FLIR camera to take a picture when heated to 80 C. The back of the bed was noticeably cooler. Reflectivity of the bed made it hard to get a good picture so I lowered the bed and then un-mounted the heat plate and flipped it on edge. I took a few pictures which show that the plate is 7 C cooler towards the back edge (which is the bottom edge in the pictures below.

The front edge of the heat plate is 92 C and that seems to be fine but I think the gradient causes stress on the print which causes it to warp enough to lift off the plate. Has anyone else experienced this? Any ideas of what to try? (BTW - Before this experiment I thought it might be warped glass so I ordered what was supposed to be flatter glass from a company on eBay and it was way more warped than mine).