Another option for you may be to change the pattern repeat to 10 sts instead of 11. Play around with this repeat: sc, ch 3, sc, ch 1, ch 3, sc, etc. Work the tr2togs into the ch 1 loop. That should make the center of your shells tighter. You might also want to play around with tr2tog, ch1 to tighten the shells themselves up.
Just a couple of thoughts since I slept on it.
Good luck, and if you play around with the stitch counts do let us know if it worked for you!