I wouldn't use epoxy alone. Cut a piece of something and glue that in. Many plastics will do nicely, but if happier with metal, do aluminum. Not wood or any composite that can absorb water (e.g. glass fiber).
I'd get a plastic specific epoxy, and if you can swing a couple extra dollars and a few days of drying, G-Flex is probably best for this. It is used for boats and things like that made of GFRP and CFRP so is formulated to stick and it never cures rock hard, but maintains flexibility along the lines of the stock materials. Even when dropped, or gets very hot or cold, it won't crack off.
If you want to fill with epoxy alone, I still would try G-Flex, but whatever epoxy you use, fill the spaces with microballoons. Easily available in small quantities for model making. They are tiny, tiny, tiny glass beads that are used in this case to add volume, but they stick perfectly to the glue so reduce the strength not at all. (If wondering, they are the same thing that makes retroreflective signs work, like ScotchLite, but they won't make the stock shiny the way you are using them).
