Child Bunk for Harmony / Symbol / Symphony

Hi everyone

After much research I've decided that the Harmony layout sits my needs - I'm a single Dad of two children and I like the sofa turning into *another* rear seat with seatbelt layout.

However, I'd want the upper child bunk fitted.

I've noticed the odd Harmony or Symphony with one fitted. I'm guessing it was an optional extra.

So, could I make one? Are the upper sides the same in all vans (i.e. strong enough?)

Do these child bunks ever come up for separately?

Thanks all.

Phil