Material to Put In a Digging Box


I am going to make a digging box for my rabbits. I just wanted to ask for suggestions on what to put inside. I'm thinking of buying organic soil (without anything added) or sand. We used to live in the Middle East and my bunnies liked digging in sand in the garden there. However, I've heard that sand is not recommended in case they eat it. I've seen a product called Bunny Nature Digging Fun Sand Mix advertised on 

I'd be grateful for any recommendations. We are now in the UK. 
Thank you

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