sorry, but i'm a little confused-how big is your container? if you are doing just a little soap at a time, i like using the KOZY SHACK individual pudding cups. they make a 2.5 x 1.375 soap that's perfect hand sized. and weigh about 4 oz each.
plastic pet food, dairy case items and the baby food isle has very interesting plastic containers. also silicone muffin pans.