I was dying for a proper wooden mold (not in my budget at all!!!)when I went to a craft store that sells all sorts of wooden boxes and I found for 4.99 euros a long rectangular wooden box with several removable dividers!! If I leave the center divider in , each side of the mold makes ten regular sized soap bars!! Do you have the mini toblerone chocolate candy? It will make great triangular embeds...just pour the soap in and peel the cardboard off a couple of days later. If you like small square bars use a retangular kleenex box and cut the soap length wise then across into the size square you desire. We have a mini ritz cracker box that makes a lovely rectangular soap bar. Look at different ways to cut the cardboard box like the side where the products name is instead of pouring soap in at the top of the box where you open it. A cereal box taped shut and re-cut leaves you the freedom to decorate the top of your soap with a multi coloured pattern. Just an idea....
I've used toothpaste cartons and toilet rolls for embeds also. I just roll up the cooking paper that comes with ready made pastry....saves buying it... (we don't have freezer paper here)and slip it into the toilet roll. I cut a bit of cardboard off another roll to tape on the bottom. (cut a larger circle then cut all around the edges towards the center, so you can fold the edges up to tape to your 'mold'. Which you can then cut the soap in half and use like a half moon embed. Don't bother lining fiddlely things like the toothpaste box.
The interesting shaped inserts in boxes of candy can make some cute guests soaps and you can use two different colours if the chocolates had two different 'levels' to it. i.e. a bump in the insert for a nut on the top of the candy....The good ol Pringles carton will give you a one off round soap mold!! I just tear it off the soap or you can use a can opener on the bottom to get it started....the list goes on forever. Soon you'll be looking at everything as a potential one off mold!
good luck!