need box frame info for runabout
 

I am planning a 19' mahogany runabout based on the old Polular
Mechanix Riviera plans. I will be modifiying the plans substatially,
including using a large, open cockpit. To maintain rigidity I want to
use a box frame for the cockpit area but I have been unable to find
any plans or information on how others have done this. Anyone have
any ideas or suggestions.