I bought some Drag Specialties 12.5" shocks. They have 16mm ID bushings which are not easily removable. I will try Verslagen1's 5/8/ inch, .035 wall pipe solution, as that seems like it would fit well. 5/8 being a hair under 16mm and minus the .070 walls resulting in an ID of just a hair over 14mm.

I also like zipidachimp's cut pipe solution if all else fails, or until I can find the perfect 5/8 pipe

Thanks for the ideas!