Sounds like you are close to having it dialed. Most cushions I see that incorporate the crotch strap have a "slot" stitched in so the belt goes through the cushion. I chose to mount mine far enough forward so the strap will mostly clear the cushion. However, lightning holes prohibit moving it too far forward. I butted the rear bullhead up against the lightening hole flange. The forward bulkhead straddles the lightening hole. (Pretty much what they do on the 9). This moves the belt as far forward as possible. I will be able to sintch the belt down with only a bit of deformation of the cushion. It also puts the belt at an angle so the crotch strap doesn't become a jock strap. Ouch! This location is really close to how Vans shows it for the 7/9. It is also at a similar angle that the Bucker crotch strap is and that is very comfortable and works well. I may have to put a pocket in the cushion yet, but I am holding off to see how I like it the way it is first. My stick slightly rubs the cushion as it is when fully aft, so I think having the strap pull it back a bit might be ok.
You might consider modifying the design by putting the belt on the forward side of the bulkhead. You could probably even put a few spacers in and move it even farther forward without compromising too much. It wouldn't take a lot to put a flange on the top of the bulkhead also, but pretty soon, you are almost back to Vans design.
Fun stuff.