I want to get into designing and implementing through 3D printing my own custom iPhone and iPad cases. The designs I want to do would actually be very plain and simple, but unique because there is one very critical flaw that I have found in all the numerous case designs I have seen. I have not found one case for either iPhones or iPads that addresses this critical issue that I wish to capitalize on. I have a lot of experience in 2D vector art with Adobe Illustrator, but none in 3D modeling. So I want a recommendation on a very user-friendly 3D modeling software program that runs on Mac or the iPad. Additionally, since I'm essentially modeling an object that is dependent on the physical design of another object (e.g. case design is dependent on iPad), is there a way I can download for free existing iPhone and iPad 3D model files to build case designs around?