I use very thin stainless steel wire.Dont use galvanized, it will break when you twist it together. 1 piece on each side in the back, and 1 piece in the front.I drill 2 very small holes in the board where I need to put the wires through.Through the board, through the holes in the skull, back though the board, twist together and staple on the back of the board. If done properly, it will be extremely hard to see and is very strong. I also use those round, colored felt pads with adhesive on the back, and stick one at the points where the skull contacts the board. I learned this method from a buddy who does custom skull work.