Now those little screen tabs, I personally don't use them. Each case has slightly different tolerances so it can make it tricky to get the screen in. It's all included in case people have wonky screens after closing the case.
I would try without the magnets also, I found the game cart sits fine without them. The other option is use some tap to stick a battery underneath the back button board. The reason I say tape is that sometimes glue comes lose over time. You can then use the printed part that you were using originally on the back of the back button boards inside the cartridge.
I've added a shot to hopefully explain things more clearly.
SpoilerShow