Most mosaicists are required to mosaic their Stobie pole mosaic on a board. Here is a video how one of the panels was done in a workshop with Tamara Froud. 'Speed mosaicing"
We use thermasheet - it is a lightweight backerboard, cement coated and with excellent fire rating. Other brands are Wedi board for example. Some of the no name product might be cheaper but will not have a fire rating which is important when you are making art for the public realm.Β