Last year I used the waist belt approach that Mike W showed. I would be concerned about velcro, as the can is pretty heavy. For the waist belt it took a little fiddling to unthread and then rethread a side strap through a buckle, but no big deal. The side strap, however, would periodically loosen so that there was too much slack and the can would then hang low and flop around, so I used a big safety pin to prevent that. After a few weeks the safety pin was rusted and at some point popped out, so have a care not to get stabbed that way.
The other issue with the waist belt thing is that if I put the pack down hastily I sometimes would clang the can against a rock or just generally forget it was there so that the can hit a bit hard. A bit alarming, perhaps, given that I had taken both safety features off the can when I started hiking with it, but never a problem there.
Also, since it was attached to the pack like that, I sometimes remembered and sometimes forgot to take the can into the tent with me (I typically sleep with my pack in a trash bag outside the tent).
Over time I just forgot it was there, but I was nevertheless happy to dump it once I walked south of Yellowstone.
And in terms of "forgetting it was there", my hiking partner and I did have a pretty classic grizzly encounter and we both literally forgot we had bear spray --- the situation worked out fine, but it wasn't until quite some time after that we compared notes and realized that neither of us had even thought of the spray. Maybe if the encounter had been within days of starting to hike in grizzly country we would have remembered, but this was three weeks into the trip.
Despite the issues with it, I would use the waist belt approach again if/when hiking in areas where grizzlies abound.