I've taken a spade bit and ground it down to a needed size - and, you
can make it a taper bit in the process. Drill your half-inch hole,
then enlarge with the spade bit. If these candle holders are for your
own use, you could make the half-inch hole, cut a kerf, insert/glue a
bit of old hack saw blade and use it to cut teh candle bases down to
size - I think they sell something like this for making dowels.