We use chimney tile square tubes over hydrants.
We have filled them with any material for insulation, wadded-up newspapers, commercial fiber batting insulation, whatever you have handy, we tape over the insulation and around the hydrant pipe on top of the tile tube and have never had one of those freeze, in the most exposed locations, in the coldest of winter.
We have made “sleeves” for hydrants out of a length of car inner tube placed over them around the pipe, filled with insulation and taped on the hydrant.
That worked very well also.