I live in Toronto, the bike-theft capital of the world, so spending a lot of money in a bike could be a risky investment, but we like nice, functional stuff, so I came up with this rear carrier made almost entirely of recycled/ re-purposed materials. The only things I bought are the 4-inch L-brackets for about $3 dollars each, and the whole structure can hold the weight of a 70 Kg dude or dudette.
Step 1: Save the World / Gather Your Materials and Crappy Tools
Go around your neighborhood and see what you can find. If you live in a G-7 economy, you can always rely on people's consumerism to provide you with really good stuff. In my case, an apartment had just been vacated and a bunch of stuff was just lying on my backyard.
You need screws with nuts and washers, two 4" L-brackets, a piece of wood at least 4.5" wide (length should not be a problem), two 2" construction screws and an aluminum crutch. Tools include wrench/ pliers, screwdriver, saw and drill (never get that $20 drill from the grocery store, or you will regret it).
Step 2: Prep Your Materials
Chop the wood to less than the diameter of your wheel size (whatever that is). Remember that you will be using crutch pipes to support your load, so they should be as vertical as possible. Sand the wood
Step 3: Prep Your Materials
Using a pipe cutter or metal saw, reduce the crutch to two lengths.
Step 4: Figure Out a Way to Attach Your "L Brackets" to Your Frame
If your bike has caliper breaks, they are attached to a metal plate and you may need to drill a couple of holes to attach your L brackets. Fortunately, my bike had these holes already, so I was very happy. -Make sure the L-brackets won't interfere with your break mechanism!
If your bike has cantilever breaks, you can come up with a new Instructable :)
Step 5: Install the L Brackets Onto the Piece of Wood
Using the brackets as a template, mark and drill through the holes required to attach the L brackets onto your piece3 of wood. My brackets had 2 holes in each side, so I drilled 4 holes. The separation between these holes is dictated by the separation between the holes on the frame. Make sure you use those washers.
Step 6: Adjust L Brackets for a Good Fit
Remember that the plank of wood should be attached to the frame in a horizontal fashion, so that the stuff you're carrying is prone to stay on top of it. Step on the L bracket and bend it a bit, try it on and bend some more -if required- until you're happy.
Step 7: Attach the Piece of Wood to Your Frame
Using the L brackets, screws and nuts, attach the plank of wood to the frame -Nice and horizontal, I hope.
Now take a piece of the aluminum pipe you recuperated from the crutch and attach it to the frame first, close to where the back wheel axle fits the frame. Hopefully your bike has these holes built-in. Use washers, even if you have to go to the toolbox.
Preferably, the aluminum pipes supporting the wood plank will be parallel to the down tube, so keeping the position, mark the spot where the tube intersects with the plank and drill. My tubes are a little off, but still work; yours should be superb!
Step 8: Attach the Aluminum Tubes to the Plank and Trim Excess Tube.
First, drill the wood with a fine bit so the construction screws won't crack the plank. Ideally, the holes will be perfectly aligned and the tubes will be parallel to each other and the down tube. Now trim excess tubing.
Step 9: Oil Wood and Paint Aluminum Tubes
I had some margarine at home which, I've read, is some synthetic grease anyway, so I don't eat it anymore. I applied it to the wood using a napkin. I got my paint at a hardware store in the automotive section. Ride away!