If done right, you would have a gap immediately surrounding the o-ring. You would not have a gap where the top met the fins. In this situation, you need enough compression on the o-ring to form a water-tight seal but no more. You basically want to crunch the o-ring until the top hits the fins and that's it. At this point, the pressure from compression must seal the joint. If you continue to tighten further, you'll begin to bow the top down at the edges (outside the ring) and up in the center (over the fins). The larger the o-ring diameter, the more "forgiving" the junction gets. Softer o-rings are also obviously more compliant, but this means less interface pressure for a given deflection. It isn't exactly rocket science stuff, but you do need to understand how it all works to pull it off properly. Hollow o-rings are often effective because they offer decent resistance over a relatively wide deflection range up until the center is fully compressed.
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