You can use styrofoam circles - cake dummies aren't anything different. You can make your dummies yourself. If you can order them, it isn't mostly worth your time and the cost, but you can make them.
Either buy styrofoam, cut (let cut) circles. You can stack and glue them together.
Second possibility (heavier) is to use plaster of Paris. Mix perlite in it (can be bought in garden center or probably in home depot) - this will make it lighter.
You can use paper mache as well. Tear newspaper into small pieces and soak in water 3-4 hours, blend in the blender until paper is mushy. Drain and pack into pan: Using a terry cloth towel, press paper into all the crevices and absorb excess water. Mold a shell only, by pressing the paper mixture into the bottom and inside edges of the pan. When completely dry, remove from pan. To make it solid would take much time for drying but can be done.
Another option of making a dummy yourself is to stack up cake circles/boards, cover w/aluminum foil or press & seal plastic wrap (NOT please plastic cling wrap) then ice as usual.
It's harder (at least in my opinion) to do special shapes this way but if it's just an 8" round or such it will work just fine.
You can use the spray foam used for insulation... It can be found in hardware stores. Take your pan, and cut a piece of cardboard that will act as a cover. Spray the pan with vegetable oil and proceed to spray the foam into the pan. The foam expands, so stop short about two thirds full. Place the the cardboard over the top like a lid and allow the foam to expand and harden. When it is cured, pop it out like a real cake. If it needs trimming, an exacto knife is all that is required. This is a great method especially for character cakes, etc.
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