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 know most people won't have to do this, but I made mine out of vinyl and then covered over the vinyl with green fabric. See, I have this problem where my hips aren't quite shaped like Boba's. It was one part that just wouldn't fit. It's comfortable compared to hard plastic, at least, and very adjustable.
I'm sure I have seen this piece for sale from certain dealers.