you definitly cant pack out inbetween the headset lower cup + the fork crown, because the two parts are designed to interface directly with each other, and infact putting something in between would completely remove the point of them. As said, your best bet is to get a thick crown on your forks to bump the height up, but really, this isn't going to be much more than 10mm to be honest. If you look at the bottom of the steere tube on a pair of forks, the tube tapers out at the bottom to ensure a tight fit on the crown. This taper isn't 'thickest' for a lot of the steerer so anything too much is going to be very unstable. Sorry, but theres not much you can do apart from stem stackage or bar + stem combos.