Here are some options that I would personally consider if I wanted a large number of drives.
If you need up to 11 drives only:
1. Fractal Design Define 7 -- accomodates up to 11 mechanical drives in a neat stacked fashion.
or
2. Fractal Design Meshify 2 -- accomodates up to 11 mechanical drives in a neat stacked fashion.
If you want to "go large" one day, and extend beyond 11 drives:
3. Fractal Design Define 7 XL -- accomodates up to 16 mechanical drives in a neat stacked fashion.
or
4. Fractal Design Meshify 2 XL -- accomodates up to 16 mechanical drives in a neat stacked fashion.
I don't think you can go wrong with any of the above.
Hope that helps a little.
EDIT:
Ah, I missed the requirement for an "XL-ATX" motherboard.
Just checked on Fractal's product pages, and it appears that for all the cases above, they accomodate up to the following sized motherboard:
Fractal Design Define 7 and Meshify 2 -- up to a "E-ATX" (max 285 mm).
Fractal Design Define 7 XL and Meshify 2 XL -- up to a "E-ATX" (no maximum measurement provided).