Here's mine. This isn't exactly a substrate cart, but more of a moving/infeed table for my table saw. Great when working with 3/4 MDO cause dat chit is heavy. I don't have the flip out bars that act as a lip to hold the material installed yet. This basically works as a portable worktable but will be able to lift the board 12" off the ground and then it flips the substrate up flat... exact same height as my table saw.
When you're done with it, you kick the support and it drops down to the horizontal position. I have the pivot bolts tight enough that it drops slowly.
I have two solid casters mounted underneath and two wooden posts. I did this instead of swivel because I wanted it to stay put when I was sliding boards off of it.
Since these pictures were taken, I installed a flip up arm so I can steer it easier when I push it around.