Yep. Another source of shims is old burnt out or otherwise no longer useful AC adapters - chargers. Years, ago, I had a Mighty Mule gate charger/AC adapter fail so I took it apart to see why. Apparently lightning got it. I did notice that the "E" and "I" core leaves were easy to separate. To my astonishment, they were exactly .020" thick! They make great sacrificial shims! You can easily get a hundred from one transformer - plus you can sell the copper wire! Most people just throw them away.I found a use for old brass key blanks and keys.
If you dig long enough, you will find some brass that is flat/rectangular and you will have feeler gauges for the extractor to breech face distance.
You may look a bit odd in some old time lock shop going through key blanks with calipers. I like flat gauges instead of round for that purpose.
I had thought about buying a few feeler gauges. They don’t go up to the dimensions required but leaves could be super glued together as stacks. (Once the rivet was out).
I measured a few keys and was done.