Box balls in a raised wall

Im moving this Monday. The house is close to the road and has a stone wall about 30inches high with planting at the top. Its looking a bit scruffy. Id like to plant 5 smallish box balls being sold in a local florist/grocers that look excellebt for £20 each. I though if a kind of gravel on the top but what kind of soil or compost would be best,?  It is northfacing