Hello. Welcome to Just Answer.
To prevent the plywood top from sagging over time, you can simply purchase a couple of extra 2 x 4's, and run screws through the bench top into the edge of the 2 x 4's.
Once your top is cut to width, space the extra 2 x 4's evenly underneath, and turn them on edge to get the most "anti sag" strength out of them. Make sure they are long enough to support the entire length of the bench top.
The ends of the supports can terminate into the boards he installed horizontally along the ends of the top surface. You could run extra nails or screws through the sides of the end boards, into the ends of your extra supports for the top.
He says in one of his posts that he installed one extra 2 x 4 underneath for this purpose, but you can't see it in the photos.
So Just running a couple 2 x4's under the long way of the desk will do the trick? Or should the top be sitting on box with 2 x4 cut in between.
You can do either one. The answer really depends on the look you are going for. The edges and ends wouldn't need extra support. If you don't want to see the supports, you would pull the front one toward the middle, away from the front edge. Will you be using the 4 x 8 sheet for the top at its full width?
Yeah I plan on using the same width
Once center support along the length would probably be enough. In looking at his photos, it doesn't appear he has a very heavy load on the top, once it's fully loaded. Will you be using yours for the same purpose (electronics on top)?
One center support, typo
Ill be using it for the same reason. Music recording. Ill be putting a 30 inch tv as a monitor on the center top shelf
If you built a box with outside dimensions matching the top surface's perimeter, then put 2 x 4's every couple of feet from front to back, the top would be rated for a super heavy load. I think that's probably overkill, unless it would be a work bench in a shop.
Makes sense. Thanks allot man.
You are most welcome. Is there anything else I can do for you?