Seal MDF the wood not the joints?

use one peice and wrap it around, take your time, use good glue, and try and make seams at the corners,

couple of tricks,,,, spray the glue on both sides (carpet, and the box itself) then let tack for about 10 seconds before you put them together....

also, make your seems with just a little tad bit extra material, then use a razor blade with kind of a shaving motion on the extra to blend the seam in.. you might have to try more then once if it's your first time, it takes some practice...

also, just the resin alone wont really do THAT much in spl, if you put a few layers of glass on it will just make the box more ridgid and that will help out a bit, but not much,   but it's still good to use the resin for waterproofing, that works really well, i have done it to a few boxes

I am not really going for SPL (although it is nice) I just like really nice clean bass that goes with the rest of the full range.

The guys at the shop where I buy my stuff said that with what I have I shouldn't have a problem hitting 140dB if I wanted to.  That is plenty loud for a daily driver.