I've made a set of replacement bellows for a Century studio camera using two layers of BK5, and it came out quite nicely. One layer of BK5 is not completely light tight. However, with two layers of BK5 bonded together with contact cement and railroad board for the ribs, the final sandwich is very thin, flexible and light tight.
OMG I was just shopping around on thorlabs site and saw the BK5 which I have used for other things i.e. curtain to convert kitchen to darkroom and am going to make a new bellows for my 5x7 I need to know what is railroad bd?
No escaping it!
I must step on fallen leaves
to take this path