Gents,
Attached is a to scale drawing in inches of the brass plume cup I had made. This cup is designed to sit within an Officer plume holder. For the plume itself, find some horsehair of the correct color. Use some epoxy and glue small clusters of horsehair together, then glue the built new clusters together and whittle away at the base until the hair fits inside the cup. Using extra horsehair and superglue, coat the outside of the cup with superglue, then place the extra horsehair around the cup until the cup itself cannot be seen. You will have to also make up a threaded rod to attach the cup to your securing mechanism inside the helmet itself. A bit complex, but it works well...