a round bottom stuff sack just takes more care to make. I just made something comparable, although my 'stuff sack' in this case is a water bottle holder.
Take desired height, add about 1 inch for top draw cord and bottom lip, maybe 1 3/8" inch total to height.
Then sew that together, I use flat felled seams on the side to get stronger joint.
After you have a cylinder, cut a circle that is about 1" larger than the diameter of the cylinder (the body of the sack, that is). The next step is the tricky part, but it's not hard, just be patient, and don't hurry.
turn the bag inside out, then pin the circle into the cylinder bottom, giving about 3/8 or 1/2 inch space to the join/seam space. this isn't really that hard, just go slow, and use enough pins to hold it in shape.
Now, slowly start sewing, I did it just using the wheel, not the motor, and guide the material to follow the circle's edge, that requires sort of keeping it a slight angle all the time, easier without the motor running I found. I suggest doing one first long straight stitch all the way around to hold it in, removing the pins as you go along, then go back and use a zig zag stitch to really lock it down. If you need fraying protection you can add either a complicated locking stitch which my machine doesn't have, but which is common, look on one of the inner seams of jeans for example, or you can use a bias tape of sorts, but I don't think that's necessary. If it's a fabric that doesn't fray, then no worries.
This was my first attempt, and it really wasn't hard, the hardest part was my cylinder was smallish, 3", a larger one would be much easier to since the curve is less.
I had known roughly how to do this by looking at a good sea to summit stuff sack I have, it has superbe stitching, and I'd noticed right away that's how they do it, I just didn't get good enough or comfortable enough to try that myself, but it works.
After you do the first straight stitch to lock it down check to make sure both edges are far enough way from the seam to not fail, fix if not.
It's not hard, just slow, but it comes out looking really slick.
Water bottle holder works too, well, and is very light. But there's no difference between that and a stuff sack in terms of sewing it.
I also double the draw cord seam fabric, ie, I fold it in under itself then sew through those 3 layers total, and also reinforce the edges where the string enters/leaves, by folding the fabric over at an angle.
After you do one like that you'll see you don't need a pattern for something so basic, a rectangle and a circle.
You might think of using light mesh though instead of cuben, you want a pot to breathe because it might be moist, at least that makes sense to me. zpacks.com sells .7oz yd mesh, hard to sew that stuff though, but it's doable.
Remember elementary geometry:
circumference of a circle (the width of the rectangle, that is, or height depending which direction you have it): c = d times pi, or: 3.14 x diameter
plus seam allowance. A compass is useful for the circle part, but it's not really critical, it's unlikely you'll get the seam curve perfectly anyway.