The rounded edges on the front aren't really necessary, but since I already had the compound mitre saw set up, I went a little crazy. The only dimension that really matters much is the space between the side supports, and that's specific to the wheel being chocked. For a GT and a GTS, the wheel is about 4.5" wide, so I made the space between the side supports 4.75". The angled piece (which keeps the wheel and fender farther back from the wall it's being braced against) is raked at about 20 degrees. The long board that runs across the back is 18".
Total time: About 1/2 an hour.