Does it join your base by sitting on four fat posts (like mine) or do you have some other type base? My top I just laid on the four posts, and secured with four l-brackets. It hasn't moved, but then iagain it's heavy enough I could have just laid it up there with no brackets. When I did it with 2x4 on edge the base was 4x4 posts, again I just used L-brackets to secure. No allowances made for movement.
If your boards are on edge, then the majority of your movement should theoretically be up/down, not side to side so I wouldn't worry about it.
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