I did not find the disappearing 9 patch all that easy. I used 9 different fabrics and I really don't like the end result - I made 3 of them and I have made heirloom quilts, log cabins, etc. - but perhaps the D9 patch looks better with only 3 fabrics - I wrote to people on the board because theirs looks great and never really got answers.
good luck - rail fence is fun!