Nope, not something to carry a canoe, but something the canoe carries.

This weekend my family and I are going on a two day, one night camping trip down the Saco River. Mum was going to purchase an inflatable rowboat(~$30) and trail it behind the canoe for storage; Five of us in three kayaks and one canoe doesn't leave much room for supplies. As soon as i heard that I started thinking, and came up with a (in my opinion) much more functional and durable solution.

This was documented as I built but, as I said, the trip isn't until this weekend. So I'll post pictures of it in action Sunday or Monday.

Update: We're back from our trip, so check step 8 for pictures of the trunk in use.

Step 1: Plan It, Start It

Make a sketch, it helps. Obviously, I planned my trailer around the dimensions of the thirty gallon barrels I had.

First, I found the center of the ends; Conveniently there were lines in the plastic already. Draw an axis in the center, grab a protractor (or eyeball it, like me) and make lines from the center to the edges at a forty-five degree angle.
Then I drew the lines as straight as I could over all the bumps and lips of the end, over onto the sides. Do the same thing on both ends of both barrels. Pay attention to which direction you draw the lines so that they line up.

The whole angles bit isn't necessary, but it makes the two halves of the final result close to the same.

Step 2: Draw Some More Lines..

You need to find some sort of straight-edge, a board for example. Use this straight-edge to connect the lines you extended to the sides of the barrel. Do this twice to each barrel. Refer to the pictures.

Measure in 6 inches from each end of each line on each barrel. Make a little tick mark.

Now find something to create a straight line on a curved surface. Try using a dog leash. Connect the tick marks to make a rectangle on each barrel roughly 15" x 17"

Step 3: CUT!

Use your weapon of choice to remove that foul rectangle. Just be sure to keep the rectangle as perfect as possible, for we will use it later. It may help to scribble at one point in and out of the rectangle, and to number the rectangle to its barrel for later.

I chose a Sawz-all for the curved lines. A circular/skill saw began the flat cuts before the greedy sawz-all took over.

Now we have created, and removed the doors to our trailer's storage.

These barrels once contained flavoring syrup for flavored water at Poland Springs, so they need to be rinsed well.

Step 4: Create Supports

After the barrels dry, the construction begins.

The lines one the barrels ran the length of the sides too, which is nice. There is also a raised ring around the barrel. These make for excellent markers for drilling matching holes. 'X' marks the sport, so drill twice on each barrel. Clean up the holes with a utility knife.

Get two rather large bolts, two washers, a locking washer and the matching nut. Two ratchets are very useful as well. Proceed to fasten the two barrels together.

Step 5: Rear Support

Grab some of the scrap wood you used for a straight-edge, cut it so it's length is equal to the width of our trailer.

Mark the center of the board. (These measurements I just came up with as I went.) Going both ways from center, mark five inches out, then mark ten inches out from there. Make corresponding marks on the back of the trailer, along the center line.

Acquire four more bolts, eight washers, four locking washers and four nuts.

Drill holes large enough for the bolts at the points marked. Then take a paddle bit at least as wide and the washers you plan to use, and drill in a bit. Drill holes into the back of the trailer barrels too.

Attach the board to the barrels. Use the ratchets to tighten.

Step 6: Doors and Such

By now you've already tidied up the edges of the doors and door frames, right? Good.

Now we need four hinges, two metal pieces, and a handful of nuts and bolts (or rivets, which I also used).

I cut, punched, drilled, and filed a single piece of scrap metal into two, which will each create a lip to stop the doors from going inside the barrels. Mark and drill the barrel and rivet the piece inside.

I marked, drilled and attached the hinges to the barrel first, then the door.

At the front and back of each barrel I also attached conduit holders to tie and bungee things down.

If you haven't realized yet, this is all just random bits from the garage.

Step 7: Hitch

Drill and cut into the top of the lip at the front of each barrel. Tie one end of a rope, a few meters long, to each hole you just made. DONE!

Weight Capacity:
-One gallon of water weighs 8.34lbs
-Thirty gallons weigh 250.2lbs
-Sixty(both barrels) gallons weigh 500.4lbs
-The entire trailer, assembled weighs 30lbs

So this should be able to hold roughly 470lbs of your stuff above water.
(Actually, slightly less because water can get in through the doors. But very close to that.)

Hey! Your sketch looks different than the final result!
Yes, I know. After I got this far I felt it was sturdy and strong enough without the braces on top.

Step 8: In the Field

First thing I did before we even left, was to drill and cut tie on spots to the bottom of the barrel fronts. I did this to try to decrease drag by pulling it out of rather than into the water.

After we filled the trunk up, we sealed the doors with duct tape before (actually after, but should've before) tying down other things.

BONUS! Once camp was set up, the trunk can be stood on end and used as a kitchen/counter-top.

If we use this again next summer, which we probably will, I'd like to use a third barrel to create a new set of doors larger than the opening. The small metal piece that kept the door from closing was something to watch out for when we packed and unpacked because everything was in plastic bags. I also want to alter somehow the front end so that it doesn't plow through the water so much, more curve and less flat.
More tie ons and a small platform between the barrels were two more ideas we came up with on the trip.