OK, How about this.
Some stainless steel cable or Bow string, Some eye hooks attached to the tube rings and a ratchet clamp/tightening thingy

. ie You assemble the scope as normal. Run the cable/bow string up and down through the eyehooks till you get back to the starting hook. Feed the two ends of the cable into one of those climbers ratchet thingies that pulls the cable/string taut and puts it under tension. I don't know the name of the ratchet thingy but I have a picture of it in my head.
http://img519.imageshack.us/my.php?i...dstringgd6.jpg
[Edit] Just Saw Robs post. Thanks Rob.
While I think I can manage some simple mods to scopes I don't think I have the carpentry skills nor technical knowhow to build my own I am afraid. BTW, your scope is absolutely stunning Rob.