If you are towing a 16' canoe, you need it to be fairly high outerwise it will be hitting the ground over speed bumps, gradient changes.
I made a trailer from a section of scrap tower scaffold by making up an 'axle' (1" box section with a M12 bolt welded in each end, plus mounting plates) and used exhaust 'U' clips to bolt it to the 2" alloy scaffold tower frame - the advantage is you can move the axle backwards for longer loads.
I've used it for my 8' pedalboat.