It's possible to craft your own Finnstick from a piece of wood (an old hockey stick used to be popular raw material). Now it's more common to use e.g. the leg of an old tripod as the basis for the stick. The upper end of the stick needs to be adjusted to each binocular model.
In the cold Finnish climate it's common to cover the Finnstick made of metal with an insulator to keep the hands warmer.
Finnstick are also commercially available and some binocular manufacturers provide an attachment for a Finnstick (or a monopod) as an accessory.