Ever done this?

So this is how my day went....

Installing a Vista 32-FB in a county building to monitor hte sprinkler system (they have both wet and dry systems)
Got flex to all the flow and tamper boxes except one and am working on the last...
First step, knock out the knock out on the back of the box - careful of the 110 bell circuit already wired to one of the switches.
Put my punch on the perimeter of the knock out and tap with my hammer...nothing...
Second tap and the box breaks off, the bell starts ringing and air shoots out of whats left of the pipe (it was the low air alarm)
I put my finger over the hole to keep whats left of the air pressure (now at just over 30 lbs) in the system until I can get the water shut off (lesson learned there - shut it off FIRST)
The water valve is chained and no key in sight. Bell ringing right over my head but nobody seems to notice. For at least 5 minutes I am standing there under a ringing bell with my finger blocking the hole. I figure If the clapper releases I will then have water shooting out of a 1/4 inch hole with a lot more pressure than the air currently is.
Finally someone notices me and I send them looking for the key to the chain holding the butterfly valve open.
No key can be found and it takes several more minutes before bolt cutters can be found to cut the chain and shut off the water.
The sprinkler guys show up later and replace the switch. The fella I spoke with said he has never seen one do that before, but theyt have recently started using plastic fittings to connect the tamper boxes to the pipes. (Plastic pipe, 3/4 od and 1/4 id)
We tapped out both knockouts BEFORE mounting the box this time....

Question: Anyone got a better way of getting rid of those knockouts? I thought of a 7/8 hole saw next time but not sure how well that may work since they tend to get grabby and either twist off the box or slam my drill (and hand) into something delicate or energized.

I would rather not go through this again....

BTW, I later found the key in the spinkler repair parts box next to the wet risers on the other side of the building (which is the first place I told them to look)
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