The shelf is made up of two identical U-shaped pieces which attach
to the shelf itself. The shelf is a simple a 4' 2x4. Each U piece
wraps around the vertical outer pieces of the main support (see figure
3). The best way to measure up this piece is to lay the pieces up
against your actual support and mark the pieces so they fit just right.
However, my measurements should be close and give you a starting point.
The idea is to make the center gap *just* the width of the main support
piece, so that the U piece can slide firmly up and down the support.
The order to build this is to build each U-shaped
piece first. Then, once you have those done, put them in place on
the main support, lay the shelf on and try it out while holding
the shelf and U-pieces together with your hand. If it's satisfactory,
put a couple screws in each shelf while its actually in place (at
his point, you have no other way to actually get the shelf on the