Yeah a Bic pen cap with the plastic tab cut off works perfect. Some amps are harder to open than others. If you need excessive pressure you should hold a towel around the amp so you don't cut yourself and/or waste any gear. Also the hand holding the amp should have a firm lateral grip with one thumb over the amp and index finger near the elbow in the glass as you bend/break the top off.
I use a triangular shaped file, the type you can pick up at a hardware store. I simply file down one side a bit and then... snap.... I've never used an actual amp opener. The file works well for me.