For stubborn cases where lifting won't work, I reach for my credit card.
Actually I reach for several of them: old ones, or expired loyalty cards, or anything else of that sort.
I clap a pair of them together back-to-back and insert them long-edge into the groove/seam where you would ordinarily stick a Mac Cracker. So far this does nothing. You can slide the cards around within the groove if you want. But once I get them to the place where I first want to pry, I get another card and shove it in between the first two. This translates an easy-to-make downward force into a powerful spreading force and persuades the case apart just a little bit at that spot.
I can do this all around the case to spread it apart less than a millimetre. If the case still won't open after that, I shove another card in between the outer two and widen the gap just a bit more. Eventually the case will be loose enough to come apart.
I like this method for a few reasons. I often have old cards rattling around in drawers, so there's no weird single-use tool to buy+store, or delivery to wait for. The cards can also flex to match the curved seam on the top of my Mac Classic. Finally, the cards are made of a relatively soft plastic that doesn't have the same risk of harming the case that metal might.