mounting rod arch - acceptable backing plate?

[fdheld34]

Gillie--
I used 3/4" marine ply and topped it off with a 1/4" piece of aluminum for backing plates underside for mounting my outrigger bases. I still get flex when outriggers are deployed (15' Telescopic aluminum). I think you would be correct in constructing some type of longer backing plate/load distribution (fore to aft)..especially for an arch. Obviously tower/hardtop has more than just two contact points so probably more rigid overall and would require less. Might use your starboard type material in combination with aluminum to create longer backing???
POST SOME PICS!!!!!!!!!
-FRED