These are examples of the type of bench I provide for access to the upper reaches of my ceiling height storage units. I vary the dimensions depending on the height required and what the benches under shelf will hold. They are incredibly strong and also make an excellent seat when dealing with things that are nearer the ground.
I make these out of 18mm Baltic Birch. The one on the right has a teak stain to match it's home. It is one of a pair that serve as seating for a child's desk.
This photo shows the grooves I put in for easy grip when moving the bench (there is a corresponding groove on the underside). They also serve the dual purpose of grip and edge awareness when standing on top of the unit.