My husband made a mold for me out of 1/2"x5 1/2" lumber. It is the width of the board, with two ends and a bottom (I know, seems like a "duh!") He screwed it together, which might not have been the best idea as it cracked a little bit. Wood glue would probably be better.
We tried to use boards that did not have knots in them, so the knots would not imprint the soap.
I believe white pine (?) is the best wood to use as it has no knots, and I've seen others on here with white pine molds.
From what I know (I'm still new too) you can use pretty much anything for a mold. The mold I have been using for my first few batches is actually a hard plastic egg tray that came with my fridge! LOL. It holds a 3-4 lb batch and works awesomely!
P.S. My wooden log mold that hubby made me is about 16" long, and would hold a 6-7 lb batch I think.
Good luck!