D&B C4 Sub

This is a commercial design. 

Why don't you pick a driver that you can afford and is available to you and then look for a free design that matches it. Or design something yourself using WinISD, Hornresp, etc. etc.

Personally I don't completely object to people building cabinets similar to existing designs.  When it comes to something as simple as an 18" reflex cabinet (with a bit of a bandpass horn thing going on), there are only so many different ways you can do it.  I can't say too much, my twin 18" subs are essentially two D&B QSubs stuck together.  However, the key point from my point of view is that I've never claimed they share anything with them other than external dimensions, are loaded with different drivers and run as part of a completely different system.

Back to the original post.  As you will have realised from the reaction of other members, you won't get any help here copying other manufacturers designs.  However, if you want some help selecting a driver which works in a compact 18" reflex cab similar to the 18LW1300, that we can probably help with.  You will need to do some work yourself though.  I suggest you look at loudspeaker simulation tools such as WinISD so you can compare other possible drivers with the 18Sound.