David,
much appreciated. I'll look around to see if I can find something suitable for the smaller model I'm working on - I'm only using #10 size bolts. I've been wondering about nickel sized paper tubes used by banks for stacking coins. It's a matter of seeing whether they are sturdy enough. I suppose I could always harden them.
I'll drop you a line if I'm in
need of assistance 
Ever seen
Wallace and Gromit?
Thanks, David.