Find a bolt in the garage that will span the diameter of the can. (Mine was 3/8, you want it strong enough for some good wacking).
The bolt had threads the whole length and I didn't want to mar the top of my porta filter...
Wait, look at the brass tube I just found, it fits perfectly on my bolt!!
Voila, done and done!!
Cheapest knockout box on Amazon is over $20, this was free!!
An upgrade would be to decorate the can, or make a wood box to surround the can... Your suggestions below!!