A friend and i are currently building Zilla Fatboy replicas. These are closed back.
I read though, that when making a closed back cabinet it's very important that they are well sealed. How do i ensure this? Just make sure the dimensions are tightly fit?

Also, the store didn't have 15mm birch triplex, so we used 18mm. What influence will the thicker wood have on the tone? Will it be noticable?
Best bet would make sure they're a tight fit, but also glue them before screwing them together.

I would think a thicker wood would resonate a bit less which would be better in my opinion.