I've been using a combo of a box cutter and an xacto #11. Patience and taking it real slow is key. Also flexing it periodically while you work the blade into the sides. I think the dimensions of the older cartridges is slightly different btw. I got a little stab. but nothing major yet. I have an oyster shucking glove somewhere I should dig out…
Truzi, the motor drive instamatics are a fun type. I have the model below yours without metering. I hope you're able to fix it/ Perhaps looking for a donor camera? can't be too expensive. I got mine for 3 bucks with a cartridge inside.