Hi Craig,
A Durs Egg kit? Is that one of the Clark kits? Craig, no task to make a lock requires a milling machine, not one. The British makers of the finest flintlocks ever made in the early 19th century did not have milling machines but they had jigs and some special tools that helped. The fly detent does have value even if you are not using set triggers. It allows you to set up a very light trigger pull without risk that the sear will re-engage the half cock notch when the lock is fired. In my experience making all kinds of locks, when you get trigger pull down to 2lbs or less, you may run into trouble with a lock that does not have a fly. Making and installing a fly is not that hard nor does it require the precision of a milling machine, although that would certainly make the task easier. I make fly detents using spring steel rod the diameter of the hole I will drill in the tumbler. I cut a piece twice as long as the hole is deep, heat it red, and bend it over in a right angle halfway. Then I put it in a vise and hammer the bent section wider and flat. Then I file it to the rough shape of the fly. I drill the annealed tumbler for the fly, and then cut the edges of the "pie shaped" recess with a square graver. I remove material between those lines with a small flat graver. Currently, I use a high speed rotary tool with a tiny grinding tip but the graver worked fine in the past. Next I use a small stippling punch to stipple the recess like I was creating a dark recessed background in an engraving. Using a small pointed half-round needle file, I file the bottom of the notch until the stippling is gone. At that point I usually have a recess for the fly that is sufficiently deep and flat to work well. Then I install the fly, check its length and file the end accordingly.
dave
