I designed a sub-woofer whose cone measured...I think 22" across...the Hartley sub. My post is still here, I just checked and it'll point you to the plans and pics for the larger Imperials. The most important design characteristics are going to be the woofers you select...and as Phil brought up in the thread, the throat size and compression chamber size. Other than that...scale the cab up...

But I did build one of these, I have listened to it...and it is one of the best I have heard. Good luck!
https://www.decware.com/cgi-bin/yabb/YaBB.cgi?board=Imperial;action=display;num=...

BTW, this sub is in a 20x40foot room and it is loud. Are you sure you need two?