advice on how to ease chickens into pen

My chicks have had free run of the garden since we got them but we had reports of foxes in neighbours garden midday so for when we aren't at home we have built a large pen and put 2 raised beds in one grass one bark. The problem is after a cpl hours they start pacing up and down and whining to come out. Any advice on how to get them used to it as don't want them waking the neighbours. We wondered if blocking the bottom part so they can't see out would help? Thanks for any help
 

They will get used to it in time.

I think Aidy has hit the nail on the head there.  Occupy their little brains - doesn't take much honest 
 
Hang up some greens for them to peck and scatter a bit of their feed over the raised beds so they have to scratch for it. You could also add a dust bath area as most chicks will spend ages dust bathing given the chance.

Mine come out for a range about the garden but take themselves back to their run after an hour or so.  Any stragglers are soon legging it back if I shake the mealworm tub.  Chickens really don't mind being in a run so long as they have stuff to do