When I met Fred McDowell one evening 35 years ago, he endorsed the string-on-fire plus cold water method, followed by smoothing the edge on a concrete sidewalk.

The hard part is finding a bottle whose neck isn't too tapered or otherwise shaped inappropriately. Fred recommended Gordon's Gin.

I never tried it, because I had a pill bottle that worked perfectly well for me at the time. (I preferred the sound qualities of glass to brass, at least on my acoustic dreadnaught.) The closed end of the bottle didn't seem to hurt anything, and had the added virtue of allowing me to use my slide to contain my fingerpicks -- two metal fingerpicks dropped into the bottle, with the plastic tumbpick clipped over the rim to act as a "stopper" keeping the two metal picks inside.

Unfortunately, all the little cylindrical pill bottles these days are plastic, without the acoustic properties of glass. I've seen replica pill-bottle slides on sale at a music store -- "authentic Duane Allman model" Corecidin bottles. (That was the name of a popular over-the-counter cold remedy that came in a perfectly ring-finger-size bottle -- probably not the only medicine to come in that standard bottle.) Except -- the neo/retro/overpriced current product doesn't spell the name the same way as the '60s era cold pill -- its' something like "Corasyden" or something.