I played fretless guitar for a few years. You definitely want the hardest fingerbaord material you can get. I was never satisfied with a wood fingerbaord and eventually got a Viger Surfretter with a Brass fingerboard. I hear good things about glass and have been thinking about building a fretless with a carbon fiber epoxy fingerboard to see how that works.
A lot of your choice will depend on how you want to play. If you want very precise intonation you are going to need to play with your fingernail like a Sarode player which lends itself to a metal or glass approach. Playing with just the fingertip will give you the same "spongy" intonation that you hear with fretless bass but the effect may be more pronounced due to the guitars shorter scale length, in this case go for the hardest wood you can (ebony, ipe, etc.).
Whatever you do your going to need to press down on the string a lot harder to get a clear note. Good luck with what is one of the most liberating experiences a guitarist can have.