Heineken Kegs...New shape?
An alternative to removing the entire cap is to use a score & tear method to cut out the centre of the cap, leaving the rolled edge in place to provide strength.
Rest a sharp-tipped craft knife on the rolled edge, and run the blade around the vertical wall of the cap, about 0.1" down from the top. Keep scoring gently until you've made a pretty deep score. Then carefully cut from the centre of the cap the the edge, and very carefully just cut up to the scoreline, taking care not to puncture the side of the can. Now you should be able to use the handle of a teaspoon to press the body of the cap into the can.
To make a lid, cut the entire cap off another can with a side-cutting can opener. Do this with contrlled pressure so the cutter only cuts the side wall of the can. The rolled edge and entire cap should come away in one piece.
(When I cut up red bull cans to make burners, I always remove the lid in this way, before using the can sidewall for the burner inner wall. So I have a box full of cut-off lids awaiting recyling. You might call it obsessive, but I'd like to think of it as 'practising'...)