Need advice for building a PA like subwoofer for upper infra frequency

[Booger weldz]

A friend made this. I think it’s the same driver? It’s kinda like yours a little bit abbreviated?

 

[weltersys]

I got xmax ratings of 12,9 mm onedirectional. i got around 3,3 mm space until i would hit the wall with my cone. Afaik my subs move quite a bit in the t-line i already have. I wonder if 3,3mm space is safe enough for the cone.

If the baffle material thickness is 19mm, and the baffle cutout diameter is 354 mm (13.94 in), the "rear mounted" cone has at least 6mm excursion potential past Xmax.
I always prefer to design for Xdamage/Xlim/Xmech (28mm for the Faital Pro XL1400) before contact, the "V" board at the throat of the lower driver looks like it would allow around 38mm excursion.
Did you mean 3.3cm rather than mm?