Thanks pbromaghin & zsas
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The test bellows.. not a great result.
I used info on the constructing found on the internet . I may have got the measurements slightly wrong as the slats ended up very wide and the total width of the bellows was way over size .
Also the choice of material may have been not the best. I started by using home theatre black out material, it did stop the light but was very thick and would not fold that easy. Then for the outer cover I used some thinner black polyester material not as thick as the HT stuff but still not that good!.
Conclusion ..don't use the internet figures..try using something about the same size as one of my folding cameras.