For 48" wide burlap, I'm buying 2 yards per panel. That's 1 foot all around to cover the sides and enough of the back to staple to the frame. Fwiw, I'll be building the frame from 1x2 furring strips, exactly the same size as the panel. It should add 1-1/2" to the depth of the fiberglass. The frame will be covered with cambric (dustcover fabric) or weedstop on the fiberglass side.
Isn't it amazing how expensive the fabric becomes once you start figuring out how much you'll need?! In my case, I determined how I was going to build the panels and then estimated how much fabric would be required to cover what needed to be covered. I found a fabric I liked and it was conveniently 60" wide. It made a huge difference to have that extra width. In some cases it allowed me to use a single yard per 2x4' panel (depending on how I made them). In other cases, I still needed 2/3s of a yard to cover the back and one full yard to cover the front.
I guess if I were to generalize, I'd say 2 yard per 2x4' panel is probably a good rule of thumb.
If I were to cover front and sides of a hypothetical 2’ x 4’ x 4” deep panel, I’d want 24” for facing + 8” for the sides + about 3” or 4” extra for wraparound to attach it to the frame, giving you about 35”/36” needed for the width. Height-wise, 48” for facing, + 8” for both sides + that same 3”/4” comes for about 59”/60” so you’re going over every time if you tried to orient the panel on most fabric perpendicularly.
If you can find extra-wide roll of fabric you may be able to maximize the material better, but with a lot of fabric yardage running 46” to 56” in width, it’s easier to just deal with having scrap and getting a single piece that you know will fit and look right.