Step 2: Bend the Curtain Rods (or metal strips from filing cabinet... see materials update).
You are going to take your curtain rod and bend it into a U at the appropriate height. The first leg becomes an L, with the second leg making it a U.
First, you need to have a basic idea of how long the supports need to be.
Find the hole in the frame near the hub of the rear wheel. Put the end of the curtain rod over the hole and hold the rod vertically. About an inch above the tire, mark a line. This is the appropriate height. Do the same thing on the other side, using the other end of the curtain rod. Where you marked the lines is where you will bend the rods.
The space between the marks is where the deck will attach and rest. It should be nearly four inches to offer support. If it is much longer you will need to trim the length of your rod. Measure four inches from the first mark: this is your new bend line. The amount the new line moved from the old is what you need to cut from the end of the rod.
Now, take your rod to the vice and clamp it in at the mark. Pull the rod towards you until you have a right angle. Put it in again at the second mark and bend it there, too. You should have a U.
Things to Note:
Most curtain rods have that little bend on the end. If you need that length (I did) you need to bend it straight. Do it as you can always cut it off.
I used a large table vice for bending, but you could probably use the edge of a table. Some of the folds might tear the rod. As rods are pretty much disposable, you can just get a new rod and start again.
I also flattened out the ends and the cross piece where the deck board will sit. It makes things flush when bolted down.
Update: If you are using a metal strip from the filing cabinet, note that they are shorter than a curtain rod. You'll need one for each side. Bend them into an L and overlap them as you attach them to the underside of the board. Very strong.