There are some cases that are pre-formatted to fit the Genesis, but most are UGC. The ratios of the UGC and the Genesis clam-shell are practically the same so all you really need to do is re-size the UGC cover in Photoshop.
Just download the cover you want, then open it in Photoshop. After that just re-size the image to 3188 x 2020 and print it out. Should come out the exact dimension of an official genesis insert, it'll will also fit on a regular sheet of paper as an added bonus.
Hope that helps!