After fiddling with it for a while, I was able to make a custom object with the right dimensions, but it does not have the extra blue plastic at the top like a normal case; it is just the cover area with a small outline.

This is beta, obviously, and I think the collections portion could be fleshed out more. There is also no option to adjust the lighting, and if you rotate one of the objects to show the reverse, it is very dark. Perhaps these issues will be corrected with future releases.