I've seen this activity before and had been meaning to try it at home. It was surprisingly easy and wonderfully satisfying for Finn.
I had some bars of hotel soap (left over from my days working as a Flight Attendant), but any translucent soap would work.
I helped Finn cut the soap into small chunks of different shapes and sizes, and gave him some toothpicks to construct his masterpiece.
It was a good activity to help him learn more about object placement, balance and working in three dimensions.
I'd recommend this activity for over 3's as the toothpicks are quite sharp to work with.