Great to hear your learning a new skill. In the boat ive tried trays, plastic sheeting etc to keep the boat relatively clean but now i use one of those large soft tubs with 2 handles on them, used to get them from the reject shop, they stop crap going everywhere in the boat and also contains any prawns that may drop out of the net. Also while waiting to throw again I just hold the net over the tub.
I don't see why that wouldn't also work from land to help contain prawns and stop picking up more debris from the ground, some grasses grab and hold the net.