Installing built in Shower Mixer

Hello all, I am planning to install a built in thermostatic shower mixer. The wall is a plasterboard/timber affair and after drilling some exploratory holes I find there to be quite a big void in the centre between timbers. The wall is about 5 inches thick
I think I may run into trouble on this one - what do I fix the valve to? I'm thinking I may have to put in cross bracing timbers or maybe just use the 15mm pipes as anchors.
Anybody done this or can advise best approach.
Many thanks
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