XL Firebox Issue

Made a trip to the Corporate store today to get my XL base replaced due to a crack that ran from the vent all the way to the back. The Corporate store team handled the warranty claim and were just as helpful as can be. My issue is that the fire box ring (lower portion) rocks from side to side in the new base. (did not have that issue in the previous base). I have shifted and repositioned to no avail. When all the elements are in place the grate, plate setter rock back and forth. I am very hesitant to shim it with any foreign object. 

I would be most grateful for any guidance or sage advice. As you may know its kind of a pain to handle that base to much.

Thanks!

shaw988