Turn it into a reverse flow. Your uneven temps will no longer be a problem. Insulating it will make it more efficient, but will not necessarily make the temps even from one end to the other. The heat and smoke wants to rise as soon as it comes out the burn box, the only way to get it to the other end is with a full length baffle and then draw the heat/smoke back toward the fire box and accross the meat by moving your smokestack back right above the burn box. I had a similar problem as you do until I turned mine into a reverse flow.
Team: Smitty's Real Pit BBQ
Superior SS-3 - Courtesy of Gainesville Jaycees and Superior Smokers
2 Custom Built Reverse Flow Stickburners
Weber 22.5" WSM
Weber 26.75" OTG
Weber Blue 2000 Stainless Performer