I haven't actually done this yet, but will someday...
I use an upside-down Weber kettle (NOT its lid) for my UDS lid, so it's basically a hemisphere. There are beach ball patterns on the 'net, including some that are calculators (you plug in diameter, no. of panels, seam width, etc. & it spits out measurements). I'm keeping my UDS in the garage until I get a decent cover -- but I'll eventually cut out the pieces (a rectangle for the body & 8-10 curved segments for the rounded top) from appropriate material & have an upholstery shop sew them together.
I'll post pictures then.