if you took this design and in each of the two current areas added two more cross supports, 1x2 is sufficient size, then you could use 1/4" plywood for the counter top , 1x2 is sufficient size, then you could use 1/4" plywood for the counter top. But where you might want to drop in a since I would add a second layer around the rim of the sink to act as a doubler to help prevent flexing which will help make sure no voids develop in the caulking or plumber's putty used under the sink rim. I made my countertop with 3/8" plywood but I have a heavier stovetop that is also a heater powered by diesel. The countertop on the other side functions as a workbench. So my use is not typical as I have more weight on the counter than the average van dweller. The point being you have to design to support the loads you intent to put onto a cabinet or countertop. Lots of ways to do things and if you don't have a need for heavy duty cabinets then build lightweight ones.