I have found the area the lid will go on needs to be flat, to lay it out correctly. If it is rounded it is hard to get things drawn or dimensionally correct on the rounded surface. Also if it is rounded, to have a properly fitted lid the bottom of the lid would require to be rounded as well, & that is another whole new can of worms. Not that it cannot be done, it is just allot more difficult to do.
Give us a photo to look at so we can see the problem.
Keith Lisle