That won't work. 2 reasons, the lower holes are in tubes, if they weld even slightly off a bit, that will make the cover a bitch to remove. Second reason is the holes in the cover are smaller than the inside of the tubes. That is so you can get a socket head screw in the tube. It also means the inside diameter of the tube has to be larger than the hole in the cover so you need a way to center the tube on the hole. If you use a stud, it has to be custom made with a 5/16" threaded section that enlarges to the ID of the tube to center the tube on the hole.
I also think that you may find that a thumbscrew won't pull some of the less that perfectly flat covers down to the jig.