How to cut a counterbore for chainring bolts with simple tools There are some 5-bolt 110mm BCD double cranks out there (FSA Tempo Adventure) that put the large ring on the inside of the spider tabs. I want to figure out a way to use whatever 110mm rings I want with them so I'm not tied to a specific, hard to get, iffy quality replacement FSA ring. What's the simplest way to cut a serviceable chainring bolt counterbore into a ring with an existing hole and counterbore on the other side?
What I did in the end was this:

The new chainring was a black Dimension with a pretty generously thick tab section. I actually went with an unramped one, but running it flipped wouldn't work because of its offset. Running it without a counterbore for the nut caused the chain to get stuck on the nuts when in the inner ring.
I used a countersink drill bit in a hand drill to cut away at the holes on the non-factory-counterbored side until the hole on the top surface was enlarged to the diameter needed to clear the nuts.
For grins, I tried stopping there and mounting the ring that way to see if force from the nut/bolt would be enough to squish down the material sufficiently to give the chain clearance. It didn't work.
To create the counterbore, I decided to get risky, and used my Dremel freehand with the baby endmill looking bit that came with it, which I think is a Dremel 196 bit. I carefully cut from the inside of each hole out to the major diameter, then went in a circle from there. The countersink bit took away half of the material that was in the way of me making this circle, whereas the bit going off course into where I didn't want to cut would require a lot more force, as would driving the bit deeper, so I was able to use tactile feedback to stay pretty much on course.
After getting it most of the way there, I cleaned it all up with alcohol, and colored the cut areas with a Sharpie. Having it all black again let me observe any irregularities and touch up those spots to better keep the surface for the nuts to bear against flat. I also tested that the nuts were sitting neatly just below the surface of the ring.
I mounted it all up and the counterbores worked perfectly. Because the crank in question is strange, I had to play with spacers under the small ring to work out issues with chain rub on the new large ring in more gear combos than I liked.