I know, if you had a workbench, you wouldn't be building a workbench. Even so, you'll need some sort of work surface, even if it isn't as stable or capable as a proper bench. The traditional solution is to throw a hollow-core door over a couple of saw horses. The advantage of hollow core doors is that they're flat, stiff, and cheap. I used a folding table and a hollow core door I had bought for a future project.
Grizzly is a company currently undergoing an upheaval. This brand was once known as a manufacturer of professional grade tools and equipment for woodworking products. Though, in recent years, that reputation has begun to slip a bit since being bought by the Chinese company SIEG. These days, whether the Grizzly product stands up to its previous standards of excellence often has more to do with what you are buying than the company itself.