Have your friend make it to exact size but the cut-out in the baffle a litle smaller so after it's glued on he can use a flush trim bit to make the baffle cut-out neatly flush with the port.

The brace in the second pic is not a substitute for 1x4 or 2x4 pieces glued directly to the inside of the cabinet, then cross-braced. It helps with bracing but moreso with getting the box perfectly square during glue-up.