Being limited by time, money, and clamps I used solid core doors that I salvaged. Commercial doors in the U.S. are generally 1 3/4″ thick and dead flat. You’ll need a table saw to rip them to width but otherwise they are a quick, cheap, and easy way to construct a bench top.
My first bench was plywood that I once used to stopper up a window so we could sleep.
We lived near a railroad, so at 3 am the sound was somewhat deafening. I built a window stopper to fit the window out of plywood with wood trim, all finish nails for fasteners. I had the BORG folks cut the plywood and I used a miter-saw on the rest, since that is the saw I had at the time.
After a few years we got out of there, the ‘cone of silence’ as we called it, became my first benchtop just resting on top of an aluminum file cabinet in the garage.