I did exactly what you're talking about. 3/4" ply for the top and then a border of 3/4" ply (about 3" wide) around the perimeter underneath the top. Theb attach the legs from the folding table. It worked fine for a while, but there were two things wrong with it. It was 'L' shaped with the cutout on the left rear portion to all for seating at that point with table in front of you.
First, it began to sag because of the unsupported section. I added some 3/4" tubular steel to reinforce that from its corner across the width of the table underneath. That has helped, but a single sheet of 3/4" ply, used in this way, even with added support, is still almost certainly going to warp or sag to some degree. But otherwise you get into really costly alternative materials and designs.
Second, the rear legs get in the way of comfortable and proper seating; and I have to place the seat carefully. A different type of legs would help that.
Result: It's usable, and in fact I use it a lot. It's not perfect. But it's convenient to keep in my shed and carry it the 30-40 yds to set it up in one of the places behind the house that I normally use for shooting on my backyard range. If I did it again I would use different legs (those were ones I had and could use without any cost), I'd avoid an "unbalanced" design that's subject to sagging just based on the laws of physics, and I'd be more careful about how the seat would be placed and how much table would be directly in front of me.
The experience was worthwhile, and (like I said) I use it effectively. But I wouldn't call it "portable" (I wouldn't want to throw it even into the back of the pickup and take it somewhere), and I wouldn't call it a real success.

😂
Oh, one advantage I forgot to mention: It didn't cost me anything since I had ALL the materials. 😂