You might even scrounge the plywood at a home construction site. I
built my basement walls that way; stuff they thought of as scrap.
Times might have changed though

The thing that has changed is the "plywood". More often than not it will be OSB
these days but they still throw plenty away.
You could still laminate up several layers of OSB with a top coat of plywood or
some other material.
The advantage of a door is the top will already be finished. My hobby table is
a door with several coats of poly on it.