Introducing a digging box = more digging in other places?

I'm considering getting N&T a diggy box with topsoil in.

I'm concerned though, as I don't want them to get into the habit and start digging at the grass in their run. Atm they haven't at all for the entire time they've been outside, so I don't want them to suddenly decide it's a good idea!

where do you get a digging box from or do you just use any box?