You can find a picture of a slide box that you can build at the bottom of
http://www.advantageathletics.com/polev ... e-off.html
There are sequence photos of the drill being done the right way on that page too. Many vaulters force both hands in front of them and jump up with a big forward lean or dive at the box while doing this drill. That's not how you want to come off the ground. The photos in that page are the correct position for doing this drill correctly.
That is built out of 3/4 inch plywood and 2x3 studs.
Ths bottom piece in the picture is the size of the bottom of the regular box.
There are 2 2x3's laying on their sides as a backing to the backstop of the box with one 2x3 across those pieces extending over the side 2x3's tying it all together.
The 2x3's are screwed into the bottom plate from below, and those screws are countersunk.
The plywood sides and back of the box are screwed into the sides of the 2x3's.
The front lip of the base is cut at an angle aimed up and sanded round like the front of a ski.
The bottom is sanded flat and has a marine varnish.
Or you can buy a fiberglass one with a steel reinforced back at
http://www.advantageathletics.com/polevault.html