Verminator MK 1 loose tank

I bought an FX Vermy 2 years age and never shot it more than 75 rounds because the rear tank has always had a little wiggle left to right. I am getting ready to sell the gun and I would like to tighten the tank. I tired tighting the 2mm allen screw at the rear frame housing. It stayed firm for 10 times of pick up and handling. I don't want to spend more money on the rifle if I am already losing by owning it. Question can I fix it by taking the rear block off the frame and shim it somehow ? Thank you in advance for any ideas.