Looking for ideas on how to model something - there must be an easy way to do this.

I'm designing a stacker that has a plate moved by a ballscrew.  I'm buying a bellows-type ballscrew cover to prevent contamination:

(Sort of like this - google Image search failed me)

At any rate - I've mocked up the representation in SolidWorks attached with the hope that when I put this in my assembly and make it Flexible, the bellows will update accordingly.  Not so.  I "get" why, but I'm not sure what to do as a simple workaround to make what I'm trying to do work.

Any ideas?