@Venice: Looks like a great mod to me! If I hadn't already owned a trailer, I probably would've gotten the same trailer and modded it similarly for my canoe.
I just use my Harbor Freight 4x8 folding trailer for my canoe. I put some cross pieces (2x4 covered with pipe insulation and cheap outdoor carpet) for towing it face down. I can also run the same type of 2x4 configuration lengthwise to tow it face up like a traditional boat trailer.

I'm working on altering the sides that I previously built for this trailer to hold the canoe on top, allowing for stowing gear under it (camping, fishing, etc). I also might rig something up to stack multiple boats or a combo of boats and surf/paddleboards. As long as none of them are longer than the canoe (14') and I stack them in the middle, I won't have turning clearance issues or need to make the tongue longer.
I pull it with a fairly new (2011 model) minivan, which does have roof rack side rails, but cross bars are ridiculously expensive (more $ than the trailer or my canoe for that matter!) and at 85lbs, my canoe isn't the lightest boat to be hefting up on top like that. Plus, I already had the utility trailer, and it's finally getting unfolded and used on a regular basis. Being able to fold the trailer for stowing is nice.
Advice to anyone using a trailer -- get a spare tire. Nothing sucks more than having to leave your trailer (and everything on it) on the side of the road to take care of a flat (or go find a spare that you should have already had).