I've had a very hard time finding a harness to fit my little Betsy too.  She
is very slender also.  The first one I bought I thought she would grow into
a little better, now her younger chubbier sister wears it.  She's grown in
lengeth but has managed to stay about the same around.  I was a little
worried for a while that she would get out of it while on a walk.  It seems
like most of the harnesses (even ones meant for ferrets) are all to big.
The may fit males better,but even for Lucy (big chub) it is on the smallest
notch.  I did finally find one that works well for Betsy though.  I'm not
sure what it's called or who makes it which I guess is'nt to helpful, but it
doesn't fasten or unclip.  It's like a long thin cord that wraps around her
front legs and tightens by a sliding button (like on a jacket cord.  That
way you can put it on and then adjust it to the right size to keep her safe
and comfortable.  The only thing to worry about is that you don't pinch her
fur or skin in the button while tightening it.  I knoe they make them for
both ferrets and rabbits and are basically the same thing, keep your eyes
open.
 
-kim, Betsy, and Lucy
[Posted in FML issue 1733]