One thing that is a common complaint or hinderance to allowing a child the ability to use messy things as therapy is the mess it makes. Moonsand gets everywhere (although it doesn't stick to your carpet like playdough), sensory tubs filled with rice, noodles, bean, or almost anything else doesn't ALL stay in the bin like it's supposed to. An easy solution for this is really cheap. It's the wonderful, amazing, so easy dollar store shower curtain.
Sorry I don't know how to flip pictures upright yet
You simply take this incredibly versitale, thick, cheap piece of plastic and lay it where you know the majority of the mess will land. When the activity is finished you take the curtain by the corners and put the 'mess' back in its container or throw it away.