Am also late to the party but this thread has given me some good ideas - I want to toss another one out there. For years I've had a very lightweight, stiff and flat torsion box bench top in use, as a "temporary" top to throw on a pair of sawhorses. Kind of fell off my radar because I've been using it for so long. It's most recent use was supporting a stack of 3/4" plywood in my shop - 22 full sheets of 3/4" plywood. It's just a 36" x 80" lauan-skin cheap hollow core door. I've been overthinking what to do for my own assembly table/benchtop, and I am simply going to laminate a sheet of plywood to each face of this door and trim out the edges in some hardwood. Stiff enough, flat enough and strong enough to do what I need. Total thickness = 1-3/8" + 1-1/2" = 2-7/8", or I could use 1/2" ply on each side and save myself an inch of thickness.
Only downside that I see is that I can't really do dog holes because the hollow core of the door is inaccessible, but I was planning on using T-track all over it anyway.
Jon Endres
Killing Trees Since 1983