There is only one frame which falls at the widest part of the hull.  It's often easier to do a full size layout drawing of the frame and set your wood on it. The final measurment of the bottom and the side frame are 39" and 16". They have to be slightly longer initially. How much longer depends on the actual width of your stock. If your wood was miraculously exactly 1.5" wide then we could trust our drawing which says they are 39 15/16" & 16.55" before the angle is cut. But in the "real world" your stock might be 1 5/8" so just lay your wood on your drawing and transfer the lengths.

Clearly mark the center of the bottom piece to help with alignment later. 

I like to cut the side pieces a couple of inches longer than 16", and trim them later. This allows me to temporarily screw a piece of scrap 1x2 across the top to stabilize the frame while all the fastening is going on.

. Here's the pattern for the gusset. You can draw a full size pattern or do something like I did with these foam board scraps. Just make two 6" legs and a 9.5" leg and connect them to each other. Instant pattern! Place them on your ply and trace the interior.

The gussets are sandwiched between the bottom frame and the side frame pieces. (see drawing) Glue and nail the assembly,  with one 1 ½” SS deck screw in each corner where the frame members overlap

Limber holes: 

These are drain holes that are found in most traditional boats, which allow water to move freely from one side of the frame to the other. They  are more important in a larger boat where any water needs to drain to the bilge pump.

I feel they are optional in this small a boat since a boat this size is often turned over to empty, or bailed and sponged. There are a couple of ways to make limber holes.