You don't need an invisible CO, you can use the loop one which may be on the Techknitting blog, and that allows you to tighten up the center. Or you can start with 6 sts and increase to 12 on the first round (or even 2nd, having a couple extras in the center may make it lie flatter). Instead of wrestling with 4 or 5 needles to begin with, put the sts on only 2 dpns and knit with a 3rd one. Then after you've done a few rounds and added more stitches, add in more needles rearrange them on the needles - it's a lot easier that way.
sue- knitting heretic