grammatical number - How to indicate possession by e.g. passers-by, mothers-in-law

I'm quite fond of internal pluralisation, such as passers-by, mothers-in-law, or even Chambers of Commerce.

However, I've recently realised that I've no idea how to indicate possession in such a case.

Neither "Mothers-in-law's" nor "mothers'-in-law" seem right.

Is there a corrct way of doing this, or should it just be avoided?