Unlike the Cokin Pro (which has been discontinued and mounted with 4 screws to the actual hood), this design will have a clamshell design, with one piece mounting behind the hood and then the front piece mates to it. It looks just like Lee's 14-24 setup and very well may be the same unit, with some slight modifications to it to fit the new hood of the 28mm. For sure the back piece will have to be different. The filters mount just like Cokin 2 in the front of the slim or 1 thick. This setup is very secure once mounted, (I base this on my experience with both the Cokin on the Phase 28mm and the Lee setup on a 14-24). The Cokin solution was not a good one IMO as it scratched the hood and the screws could easily crack the plastic. I quickly took Tim Ernst's advice and modified mine to slip on from the front.
I still have some of the old Cokin Pro resin filters and all of them at the time I purchased suffered from a Magenta contamination, so far all of the Lee filters I have purchased in the 150 x 150 size have not had this issue.
Lee makes all types of grads, solids, in the 150 x 150, just no CL-PL (needed for your AF to work) or the classic 10x big stopper. Hopefully this might get them to finally make the big stopper in the 150 x 150 size. Lee's only issue is it takes forever to get their filters/mounts.
Paul