What kind of 18 did you get?
The Big Ben is a simple box, probably made of 5/8 plywood. You can work backward from their published dimensions.
The ports are unducted holes in the back of the enclosure. Technically, the duct is 5/8 in. but there is no cardboard tube. They might be 6 in. in diameter IIRC. Also from examining one (IIRC), there are a couple 2x3 pine braces running horizontally across the back of the enclosure for stiffness.
I may have made a drawing...I'll have to check at home this evening.