The aftermarket 20/21 gallon tanks are based on the 15 gallon stock tank, the only difference is that they are taller to gain capacity. Y
ou need a 15 gallon skidplate, two straps for the side, one over the top and an extension that lowers the skidplate for clearance to the body. The demand for a replacement stock plastic is so low it wouldn't be worthwhile to make, they simply don't rust out. You can buy aftermarket skidplates for the OEM 20.
The Scramblers and Cjs used the same tanks and skids.