I would build it half the width, take out the middle supports as they are just extra weight for nothing. Also now that it is half the width you can make two of them which will fit in the back of a truck easily, then you have twice the length. Its also good because then its more of a ledge than a box, so you have to use more balance while sliding it rather than just jumping onto a wide flat surface.
Also, short boxes get boring fast, so your going to want something where you can lock in a slide for a decent amount of time.
So in my opinion build two or three ledges that connect to each other.