Google 'stretchy bind off' I think there's a page with links to lots of looser castoffs. EZ's sewn bind-off is good but I think not very attractive. Tubular cast-offs are great: stretchy and look good with ribbing. Or you could bind off in pattern. Honestly I go up at least 2mm if I want a looser cast-off but I am tight. You may find it easier and less distorting to use a crochet hook as your RH needle, then you don't stretch the stitch you lift up, pass over, and drop off.