Well, here's a very DIY, cheap an recyclable method to make molds for relatively low temperature casting.
Step 1: Materials
Gelatin -- I'm using Jello
A vessel to pour in that will hold your positive item
A refrigerator with space for your vessel
You may need:
Skewers or other support device
You probably want:
Chap Stick or Vaseline (chap stick being "edible")
Step 2: How Much "Stuff" do I Need?
1. No undercuts - that is, if the mold material goes over (or into) your positive - you won't be able to get it out (without some surgical maneuvers).
2. Avoid super fine detailing - I haven't experimented with this yet, but it seems like it would be bad.
3. Don't let the positive rest on the "floor" **
So how much gelatin do we need? Lets do a "dry" run.
First, place your positive into your vessel. Now, fill with water (you may need to hold your positive under). Next, remove the positive and measure how much water is left in the container (you'll probably be pouring off into a measuring cup).
You will need one 8oz serving package of Jello/gelatin for every 3/4 cups of water. I have not tried higher concentration (yet), but feel free to experiment as a less "jiggly" mold would be better.
** There is an exception to this rule. That is, if your positive has a flat side that doesn't matter (like the back side of a chocolate bar) - it is beneficial for that back side to rest on the "floor" of your vessel. This was the case for the Tetris Ice Cube Tray.