Mine is made up of 2x layers of 18mm OSB with a thin (4mm?) bit of exterior ply on top.
Under that is a frame of 1×4 timber, with extra frames at about 50cm intervals.
4 legs of 1×4
And then a shelf similar to the top but just with the thin ply and frame.
Square of ply on each end to stiffen it.
Very strong, ive been using it as a platform to stand on and it doesnt give one bit.
Advantage of OSB is its dirt cheap and tends to be less flexible than ply so when I cut each piece and then put then on tresles I could just screw them together without worrying about them bowing (ply on top, then flip over and build the frame and screw that in from underneath, flip over again and build the legs, pull the tressles out and build the legs and shelf).
Didnt varnish the top as if you spill solvents on varnish you just end up with a sticky mess.
Only job to do is fit a soft jawed vice to the edge and some holes for dogs/pegs.