You don't need the fliptop if you have a slim PS2, you just need to use putty or tape to tape down the switches that tell the PS2 that the lid is open. That's why it's not working, it's because at the swap magic screen you're opening the lid to switch the disc which confuses the PS2. When you tape down those switches the PS2 doesn't realize that you're opening the lid and changing discs. That's how swap magic works.