IMO you can't add enough flat material or extra cross braces to make much difference with the swiss cheese rails sleds come with stock today.
They are designed to flex so the tunnel takes less I think.
I played with mine this year and added cross braces (only a few places to do it properly) and simple side plates (I have tipped up rails on my `13, oops lol). When I was done and it was true, the rails still easily flexed in the cut outs between the braces. Only difference was it stayed out of true with extra bracing instead of springing back.
So I went back to the original flexy CAD design (as was recommended to me in the first place lol). The problem is in vertical flex (not lengthwise).
IMO solid rails, or a fancy extrusion, or really thick (1/4" minimum) 3/4 length side plates are the only real fixes. The first option is the cheapest in the long run if your riding style requires that you hit things lol.