What I did was to cut a long tube in half then make one about 200L and the other 400L and play them both in an A/B comparison, and then tweaked it until I got the sound I liked. You’d be surprised how much different the bass can sound in different sized enclosures and none of the WinISD modeling revealed this. If your really serious about any speaker project always make two or more prototypes and listen to them, I never build anything without several prototypes and it has served me well.
My Sonosub project
http://kingdaddy.linaeum.com/Sonosub/