I used to use the edge of a stone mason chisel in the whole length of the slot...it was the best fit and didn't knacker the edges...First time I took it off , it took a fair bit of force and maybe a 'gentle' tap ...but take care it's only an Aluminium container.
I have a small crowbar from B&Q one end of which is like a giant screwdirver head. Fits perfectly. Just have to be very careful when tightening up not to apply too much pressure.
I just use the body of an open ended spanner. It can be tight so be prepared to use a bit of leverage and use a new o-ring when you put it back in to avoid leakage.
