what the OP is posing is that he is building a shelf with two supports
on either end and one in the back the length of the board. looks like
this:

------------------------------------
| |
| |
| |


where the ----- side is 60" and the | sides are 24".

I made a 54"x20" shelf from 1/2" MDF for linens and such, and it
doesn't sag at all because we supported the back with a furring strip
54" long.

Since you want a lot of weight on it, it's probably going to be utility
use and appearance is second concern. I'd use OSB tongue and groove
flooring (the yellow painted edge stuff) I think it's 3/4" thick. If
you think about how that stuff mounts in homes on trusses and how much
weight it holds without warping.

You could also use corrugated tin and roll the edge over in front if
you're holding something like paint cans. ugly, but it won't deflect
even if you jump on it.

good luck.