Some painters take loose canvas, cut slightly oversize, taped to board, out in the field. The advantages are that you can easily carry many sheets of good quality canvas out, without much investment, and you have more flexibility on the finish size.
If you are using hardboard, seal the surface. I use Matte Medium or Golden GAC 100.
Many painters use Miracle Muck or Matte Medium as an adhesive for their canvas to their panels. I feel less comfortable doing this with finished canvases. My system is simply to use photographers cold mounting adhesive. I use Grafix Mounting Adhesive. So far, it has worked beautifully.