I have a couple of suggestions:
First, instead of casting on "normally", make the slip stitch loop and then crochet the stitches around that you need. That way the cast on is tighter, and you can pull it closed tight when you are done.
Secondly, and what I'd do if I was you, is try the umbilical cord cast on - essentially you make a 4 stitch I-cord in waste yarn, so the sts are stabilised, and then work from that. The waste yarn is removed and the cast on is pulled tight at the end.http://techknitting.blogspot.com/2009_09_01_archive.html
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