"THE FINAL SOLUTION" ......pun intended. Now I see what your up against. Simplest, foolproof...it ain't coming out bulkhead....
This is how I put them in. Bear in mind these pics show the NC foamed and carved out for a tracker and the tubes for the antenna. So just ignore that part.
Cut off bottom of NC . Fit some allthread to length needed to go through BP down into NC tip.
AS seen in the photo.. on the tip end of the allthread nut, washer, nut, .....when placed into position, foam poured in, held centered by the BP. No way in Hades when this foam cures, can you pull it out!.
You will gain your 4in. have space under the BP to carve the foam for a tracker if you so desire and have a removable BP to boot!
I used 2-part foam from Wildman [Pro LIne] I think it was 12.00 but PML and GL also have the equivalent products.
WARNING>>>>> when using this foam it can get rather hot....so I always place my NC in a water or ice container to prop it up and keep it from distorting.
Final shot shows how the rod is centered and the eyebolt is offset. Of course yours would be recessed 4 in. into the shoulder.. As you can see this is definitely positive retention.
This foam is an adhesive, We have used it solely by itself to hold fins on rockets. MY Gizmo, flown on K=805 and CTI dual thrust 6grain xl motors no problem!