I made my butt plate out of Sintra (any thick plastic should work). I guessed at the measurements, made a template and traced it onto the plastic. Then I cut it out, sanded the edges, and used a heat gun to curve the ends.
Then prime, paint, weather, etc.
I used 2" black elastic to attach the butt plate to the codpiece with rivets. Then I simply step into it like a pair of shorts.
I hope that helps!