Mick, are the tube rings adjustable to suit diffrent appeture OTA's. Also it may be worth while to consider a Split-Ring Equatorial mount design with adjustable OTA rings if possible.
some examples
http://www.jimsmobile.com/old_ngt6_data.htm
http://www.geocities.com/lynol1000/splitring/
some ideas for you.
Ps so many ideas and no talent to make them...