One thing I tried that kept them occupied for AGES, was to buy a couple of those little willow wicker balls, stuck in some hay or crinkly brown paper, and then push in some treats. They spent ages gnawing on the willow and shredding big holes in it, before they could sniff inside for the treats.
Use something they love and is quite strong smelling, and they'll be determined to get it out!
I know thats not really DIY as you have to buy the balls, but you could make something similar?