Changing Table Question!

Elizabeth
Little backstory: my mom tells me this story once in a while about when my brother was a baby and she was changing his diaper. He was thrashing around on the table, and managed to throw himself over the edge. My mom caught him by his foot, so he was fine.
But I started wondering, why don't changing tables have raised borders around the cushion?
I've never changed a diaper on a changing table before, so my question is, would raised edges around the cushion make it harder to change a diaper? I plan on designing and building my own changing table instead of buying a premade one, and want to know if this will work