How I cast a sheep load in plaster for a wagon

Having recently constructed an open topped cattle wagon from an IP Engineering kit and repainted a secondhand Tralee & Dingle open topped cattle wagon in PLR livery, I decided to give them both sheep loads. I bought six sheep from Motley Miniatures for £20 and six 3D printed sheep from DesignPrintScan3D for £21 and duly loaded them into the wagons, using the bristles from a yard broom as bedding straw. To me, they looked acceptable, however ........

..... someone more knowledgeable of these things than me, pointed-out that in reality, when sheep were loaded into wagons they were crammed in tightly to help ensure that none could fall over and get trampled on. So, I was faced with a problem. I worked-out that I would need at least 20 sheep per wagon to fill them - which would cost around £120 in total. Considering the wagons only cost around £50 for the pair, this seemed somewhat extravagant to my penny-pinching mentality. So, how could I fill them more cost effectively?

In the light of information which has been presented to me, I have had to remove the rest of this blog post. I am in the process of rewriting it - please come back later