Changing dimensions to an existing design is a big no, what you can do however is to build the cabinet as it is and
add an extension reducer ring so you can mount your 18" driver in to the 21" baffle, that way later on you can swap the 18" for the correct driver.
i wish Ricci published the HR screen of this design, that way you can sim different drivers in it.
but i think there is no much leeway as this cabinet need a very strong motor driver with military specs cone
so there are not many 21"s that fit the bill, and the ones that fit are already listed on post#1
My 2 cents.