The upper/lower on my VFC is solid like rock so I can't give you any exact directions but if I was in your shoes I'd first investigate the tabs on the upper where they slot into the lower and make sure there is no damage.
Then I would build up some shims with a slight wedge profile that would sit right at the rear on top of the box to snug things up. I'd pick a harder plastic material and make sure it wouldn't foul the charging handle or more importantly the spring in the mechbox.
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Airsoft, where nothing is hurt but feelings.
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