If you have access to a standard BIC ink pen, a straight plastic tube with the ball point and ink tube inside, they work very well for the tool to which you refer.
Simply remove the ball tip and ink tube from the inside, leaving the outside body (tube) of the pen. The 'tip' end fits the FX piston very well and can easily remove the piston by a slight wiggle.
Installing is basically the same as removal, except you will need a rod long enough to push down through the pen body to lightly push the piston into place, releasing it from the pen tip.
I hope that I have explained the operation well enough. It works great.
mike