Well the way Apple products are modeled in the first place to get those particular shapes isn’t tricky, your process is probably fine, it’s that they–ninja-level point-pushers–spend an insane length of time tweaking until it’s just so.
The problem I have is that the surface is always too thick. If I then pull it thinner with the gumball, the backside changes, however, so that I only end up with a straight rectangle there