That's the same way I design all of the work benches I have done with the sistered up 2x4's as legs.
For my top surface anymore I do a sheet of 3/4" th. plywood , then add a ssheet of 1/8" masonite on top of that and use double stick tape to keep it down and then trim out the edge by screwing horz. hardwood thin trim to horz. top supports leaving the masonite to rest on only the plywood and the trim makes things flush at the lip so you see neither the plywood top lip or masonite lip when viewd from teh side.
When your masonite gets cut up/dirty/used up/gouged, you just pull it up and flip it over and re tape on the back face. Add to that the cost of masonite is far chaper than hardwood or finished workbench tops.
VERY nice build tho. Well done.