I was inspired to build this when I found half of a longboard in someone's trash. It had been snapped cleanly in two. I noted that its shape would make it the perfect platform for a bike carrier; the grip tape and the curved brake at the back would stop objects from sliding off. So, I cobbled together this project out of junk I had in my garage.

Step 1: Materials

All of these things can be salvaged;
Half a broken longboard or other skateboard
Pieces of wood to hold up platform; they should be long enough to reach from the desired platform height to the mounting point you intend to attach them to at the other end, which is probably located near the rear wheel hub. I used very lame wood. You'll need to more wood scraps to make the two l-shaped brackets I'll describe later.
At least six wood screws.
Four bolts that fit the threaded points you're trying to attach the carrier to.
Four attachment points on the bike; chances are, your bike already has them. If not, you could improvise something out of hose clamps or zip ties.

Step 2: Cutting the Broken Board

Use a saw to cut off the jagged edge of the board as close to the break as possible. Sand the edge if you like.

Step 3: Brackets

Cut two L-shaped brackets out of wood scraps. the long part of the L will be attached along the bottom of the longboard, and the short part will protrude down and be bolted to the bike's attachment point (drill the appropriate hole in the bracket to accept the bolt). Make sure you put them the right distance apart.

Step 4: Supports

Get an idea of the distance between the bottom of the board and the mounting points above the rear wheel hub. Cut the pieces of wood to the appropriate length. You can use a single screw driven through the board to attach them at the top, and a bolt driven through an appropriate hole at the bottom. make sure the board is centered and doesn't rub against the rear wheel; leave enough clearance for tie-down straps.

Step 5: Finished!

Make sure everything is strong and centered,and it's done! You can secure objects to it with bungee cords or straps. As I said, all parts could be found in a dumpster. A free bicycle carrier!