If it is a bass cab you are building, this may be of help, although some principles will probably be the same if it is for guitar.
https://www.basschat.co.uk/topic/200152-1x12-diy-cab-build/?tab=comments#comment-1983824
https://www.basschat.co.uk/topic/200152-1x12-diy-cab-build/?tab=comments#comment-1983824