: Is marine grade plywood required or can cheaper grades be used and protected
: properly?

The footbrace is not a structural member of the boat, and it is easily removable, so marine grade is not required. Seal it all really well though.

: How do you determine the proper angle (bottom to top) to cut the front of the
: upright support? Is it determined by the way the other pieces fit together
: or is it a matter of how you want your feet to sit?

The angle is not critical. I used a bevel gauge and took the angle directly off the book page, and it feels perfect.