Advice on replacing 12 in woofer in Martin Logan Monolith III's

"Sorry Tim but you clearly have no experience with mating esl’s"

But what I do have is a knowledge of how it works and why.  I do understand blending panels and a woofer...
I know that you guys don't know me from anyone, but when Sumo designed the Aria speaker, they did call me for advice and way back when,  The very company that you are speaking of Martin Logan did call me and ask me to come interview with them... which I declined. 
I'm old and I've been involved in speaker design since I was 19. So rather that turning this worst and telling you what you don't know, I will bow out....
@stevepa can easily private message me if he needs help. 

 Correction on the box size it goes back 6 inches before it starts to taper

That will make quite a difference.

Best to lay the Monolith on it's back take the driver out and stuffing, fill with bean bag styro bean fill, empty all into a known litre/gallons container (I use a 20lt contaner I had) to get the exact volume.
Then Google litre/gallon to cubic ft and use the calculator to find the exact cubic ft on the Monoliths bass enclosure size include bracing, esl power supply, wires ect.

Cheers George

You would think this  measurement would be on the Internet. I've looked at MartinLogan site and around the net, but can't find it. 

No it's not, as I found. You'll just have to do it yourself as I did, wish I saved it all those years ago.

Cheers George