He could mount the supports on the bottom but only if he puts the swim platform up high. If he does that, however, what't the point? It would be just about even with the built-in platform.
I was just thinking that if you want to get really innovative (or ghetto?), you could try mounting to the sides of the boat. You could get some brackets fabricated out of stainless or aluminum plate that mount to the sides and extend back to support the platform. Like this:
You could probably get a piece welded to the insides of the supports through which you could bolt some wood. Or maybe someplace can bend the tops of the supports over forming an angle for the same thing...I'm not sure how they do that.
If all that is unavailable or too expensive you could just rivet/bolt some angle stock to the supports.
I just figured that if you can't get good support on the back, the side is wide open.