Very nice!
Since you've installed a damper on the top, maybe you can get a Maverick ET-732 and some 6ft probes and go in through there. I used to run the probes through the top on my propane smoker and it worked perfect. Granted that opens from the side and not the top, but you could probably make it work with the 6ft. probe cables and mounting the receiver to the handle on the cart somewhere. You would just need to be careful when you open the lid. To me it would be worth it because then you would know your pit temp and meat temp so you shouldn't have to lift the lid much anyway.