Ignore the woofer displacement, port volume, and any volume taken up by the plate amp...your damping material will compensate for that and then some...there's no exact formula or rule, but at least line the sides with loose polyfill, foam, or insulation. The amount the woofer will vary from the published specs will make a bigger difference than these finer volume calculation details.

Your 12" cube box should be made of 3/4" material...so 18.97L or 0.67 cubic feet is good enough.