Ottersatin:
Here's those pics of the shelf.
The first pic is of the shelf about half-way in.
You can see the 'tracks'/runners screwed to the hay-box.
The next one shows the shelf extended.
The shelf itself is reinforced by two large cleats,
and a smaller one. The front cleat also acts as a handle.
These keep the OSB from sagging.
The final pic shows the 'slot' cut out of the track.
where the shelf slides in and out of place.
There's about 3/32" side-play between the shelf and the tracks.
And 1/16" from the top of the sliding-shelf
to the bottom of the hay-rack.
Hope these have helped.
For sure in this instance,
a picture is worth a thousand words.
Thanks for looking.
grumpy.