I worked some more on the issue with the coin slots this evening and it turns out that that most coin holders out there have somewhat of a problem to them. The flutes in the holders don't really "hold" coins. The coins wiggle and don't sit at the desired angle (8 degrees), depending on the thickness of each coin. That thickness varies a lot I learned by now, being the root of the problem.
Where this is coming from is that most coins are different thicknesses and there is no easy way to tell why or how to make it better. I found some ideas to use a dove-tail bit on the Vectric forum, but nobody seemed to have this done successfully yet (ok, a challenge), there are no pictures or examples.
I felt challenged, so i took the whole problem into Sketchup to see what's going on there, looking at it sideways. To me it seems that with a 8 degree dovetail bit, at a certain height, the coins should hold at the same angle. Running a dovetail bit through a piece of wood at full depth is certainly suicide, but perhaps we can clear some of that with a 1/8th End-Mill first.
The circle in the lower right of the drawing is the diameter of a toothpick by the way.
I figured, if someone wanted to fix the coin in the slot, they could break off a piece of a toothpick and put it down there to make the coin sit steady forever.
I ran out of time this evening - to try this out on the machine, I will let you know how it turned out tomorrow, but we may be onto something that's different from the off-the-shelf coin holders.