I would not do the styro as it has several things that I don't like and adds very little. Work to avoid twist in the bottom and let the rest go. For the bad side of styro, you will likely find water creeps in under the styro and that can't be wiped out so tends to stay wet and lead to damage of some sort to the wood. Not a big loss but then the edges will also show and so I don't bother for the small feeling it gives me. Any small point that sticks up on the wood will be more likely to make the wood bow down enough to match rather than push the tank and water up.