The Canoe/Kayak Livery nearby uses utility trailers with 2 metal uprights, and cross bars on each. Can't remember if they're welded or if they use the heavy duty threaded pipe/conduit. They can put up to two yaks on each "shelf", sideways, or 1 each top-down. I think they've got 4 levels, which means 8-16 boats. I'll see if I can get a picture.
Yak4fish has a commercially built kayak trailer - very sweet. Can launch/load directly off the back at a ramp. Not sure what his capacity is - would guess 4.