If you only need to cast features on one side of your part (the back side being flat), you can make an easy mold using RTV. Here is a link;
http://www.eagerplastics.com/castingpewter.htm

If you can pull a vacuum on the rtv before it cures, it will help to remove any bubbles.

Next, there are a lot of jewelry casting metals that melt a really low temperatures....some as low as the low 200 deg F. Cerrocast is one that melts at about 300F. These should be sufficient for your part...they dont have much strength so cannot be used for structural stuff.

mike in tucson