Shoot some expanding foam underneath the foam board you already have on there, don't overdo it or it will lift the tank and don't use the low expansion window/ door foam as it dries soft, you want the one that dries stiff, you can stick a piece of rigid tube on the end of the nozzle so it can reach to the center, works great seen it done a number of times, make sure you level the tank before it sets.
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