In order to bake polymer properly you shouldn't adjust the time and temperature from what is recommended by the manufacturer. A clay craft oven offers even heating and a larger baking area and would help resolve your concerns.
Regardless of the oven you use, one solution to help protect the polymer during the baking process is to bury the pieces in a container filled with cornstarch or baking soda. Either of these materials will protect the pieces from the extreme spikes in temperature, as well as from the elements. This is also a good method to use when baking a polymer design that has structure, as the cornstarch or baking soda will offer support.
Another solution for protecting the polymer pieces while baking is to tent the tray the designs are on using aluminum foil. The foil will dissipate the heat and protect the designs inside. An oven baking bag (the ones used when baking a turkey or chicken) are a great option for baking polymer in a household oven--this option is not for use in a toaster oven. The baking bags will contain any fumes released by the polymer and protect the household oven from the baking process as well as protect the pieces.
- Tammy Honaman, Author, Jewelry-Making Expert and Educator