Building a sealed sub box

depends what subs they are.. And depends how much power you are running through the subs... If your subs are 800 MAX power and you are only running 100W rms each from your amp, then its still 200wms.
And don't go by max power.

But for sealed, I would just go standard 18mm, and do bracing whereever possible, to minimise flex, no point in 36mm if you are running low power/ low output subs.

Other thing to keep in mind is that it is pretty hard working with wood much thicker than 18mm.