I just did mine and here's what I did. I measured the width at each brace wall to wall and the width between braces then I drew a center line on the wood and basically plotted the points where I took the measurements. then I simply connected the dots and purposely cut the wood a little big(easy to cut more off. hard to put it back on). Then when I put it in the boat I simply marked the areas where it hit the sides and shaved those areas off, continuing to refit between cuts. After 3-4 "shavings" it fit perfectly. One thing to watch out for. is make sure to take into account that the top of the wood will be 3/4"(or whatever size plywood you are using) higher then where you are measuring the supports, and thus will probably be wider then where the braces meet the walls of the boat. For mine I needed to cut the wood on 45 degree angle as well because of the slope of the walls. Seamed to work pretty well for me.