hi, I'm making a dolly for my amp, it weighs 75 lbs and i already have 4 casters for it.

I'm wondering what kind of wood(if it matters) will withstand the 75 lbs, and how i should make it. (the dimensions are 20x15 if that matters)

I was thinking about just taking 4 pieces of wood and connecting them at the corners with the casters, but will this support the weight?

Any tips are appreciated, thanks in advance.
Get a piece of Plywood and mount the casters. That would be easiest. 3/4 inch would probably be fine.