We have the same FF and the same 'weak spot'. I have not yet fixed it, but I can't believe ... yes I can ... that the dealer thinks they have to do a fix that can't easily be removed for plumbing access ... my solution is basically the same as next2dixie's, but I was making it light with a 'T' made of 1/2" plywood gluednscrewed, a piece of foam between the 'T' and the shower base and a small cleat on the bottom of the 'T' to screw it into the floor. Need access, you just remove screws and take the whole thing out. I may re-think the idea of a 'box' over a 'T' .... Hmmm maybe a hybrid ...
I think the reinforcement is needed since that is a guaranteed stress point with every shower taken!
And, FYI, I found that all the holes where drains were going through the floor needed to be foam filled.
Fred n' Jeri
2012 FF X-214WSD
2010 RAM 1500 Crew Cab 4x4, 5.7 Hemi MDS, Huskey 1200lb trunnion WD, Prodigy Brake
Prion Seayak Kayaks