Back from my loud stereo days here is what I remember...
Look at the specification of the sub-woofer, and see how much air space the manufacturer (you know they pay engineers to figure this stuff out!) recomends.
There should be specifications for Sealed, Ported, and Free Air Installations.
I myself like sealed boxes, but to each their own.
Buy a box that meets the manufacturers specifications.
Then make sure you don't under power, or over power the speaker.
back in my car audio days, custom boxes where all the rage. I started to teach myself, box designs, Sealed, Ported, Isobaric, 4th, 5th, 7th order, blah blah blah, and other "audiophile" information.
Just find the cu. in. volume the speaker needs, and plan your L,W,H, dimensions.
Bigger amp wouldn't hurt.
Is your Amp Gain set right?
Does your head unit have provisions, for subwoofer control? +/- db?