I discovered something in making casts - The molds need to be about 1/16 inch deeper than wide, if casting is to be done with fill-in material. In casting coffee beans (and probably other material) the material will lift/float a little due to its lighter weight, particularly on slow setting resins.
I have about 1/16 to 1/8 clear layer for the whole lenght on the bottom of my beans cast because they "floated". I had a top on it but they bunched up anyway. My answer to this is to make the 3/4 by 3/4 mold to be 13/16 by 3/4 or 7/8 by 3/4. I want to try the red pepper blanks but I figured I would get the same thing.