I am going to say that the decision between foam and epiweb is completely up to you. Keep in mind that you won't be able to attach any potted plants to it, only epiphytes.
As far as field collecting moss, i think if you ask 40 people this question, you will get 40 different responses. Now, i personally have done this, but i collected moss from a rock in a stream. I rinsed them thoroughly, then dipped them in a very mild bleach solution, then they got another dip in a very mild hydrogen peroxide (H2O2) solution. Then grew them emersed for a week before i used them. Wood is easy, boil it or bake it. Just be careful to collect only HARDWOODS. No evergreens. Oak is safe and should be plentiful in G.B.