Frame underneath the ply top, 4 legs.
Enclose 3 or 4 sides underneath with cross-braces at maybe half of the leg height.
If you ever want to sit down at the bench for fine work then make the cross-brace nearest to you halfway back to the wall.
I'd say build it as a lump... if you ever want to have a shift around then an integral built bench is a right pain. Build it once and build it heavy then maybe join it in to the wall if it needs it for a bit of extra rigidity.
To do the frame I'd just do this but in wood, should illustrate the idea a bit better.
Even with only 3 sides enclosed at the bottom that is relatively rigid even without tacking it into the wall.
