Many casters make their own boxes from various materials. Others use the handy slot car case lids.
But I just don't have many case lids and found some inexpensive food containers. I tested the size and they seem to fit just right.
They are 4x7 and come with a lid. Not much need for the lid but the tub works great.
Just cut a pouring hole up front and then hot glued the tub to my base and poured away.
You can push the mold out from the pour hole.
I am finding these at Dollar Tree, Family Dollar and even our own local grocer. You get 2 tubs for $1.00
Just an option.