You have never measured any of this so this is pure conjecture.
That's where this whole thing started. He bought (what we assume should be) a decent design from somebody and half arsed everything along the way from too few layers to not sealing properly.
I was saying 2 18's from the get go, but read my previous reply (or the entire thread if you can stomach it). His initial fiasco was his attempt to build a design someone else sold him and was half arsed all around. Craig Butler could hand him a design and he won't get loud if he doesn't build it solid and airtight and take time to properly seal it off from the rear.
All things equal this is typically true. The extra power the 4" coil can take is pretty much what you need to overcome the inefficiency of the larger coil. All things being equal 3" coil should edge out, that's why Warden DD-Z and similar still use them for their SPL subs. That said, who knows what quality steel, magnet, parts, and design CT uses? A sub that looks beefy could always be poorly designed or use inferior materials and be utter crap. Though in this case, I don't think the subs are the weak link here.
Hopefully this means you sorted out your financial issues without having to give up all your audioz. That hurts even to think about.
Back to square one? OK. Plan for port on the driver's side, use as much airspace as you can (keeping all right angles), and as much port area as you can. Do all sides 3 layers, baffle 6 layers and TAKE YOUR TIME making sure everything is rigid, airtight (not with foam, with straight cuts, glue, and fasteners), and then take your time COMPLETELY sealing off the front from the rear.
You can very easily shrink the box or port as needed after everything is solid and upgrade to some chunkier 18s down the road if you like.