All materials are possible, but I recommend "Strong & Flexible Plastic" only!
I wrote the above for people who won't pay any more attention :-)
The two parts will only stay together with a flexible material. Printed steel is not very flexible! I don't recommend it, I got an email from someone who said that they tried and it didn't even fit together well! I felt bad for them, so I am adding this warning. Detail plastic is fine, but you need to be careful not to crack or chip it, and it isn't porous, so it's harder to paint. I haven't tried the model in a precious metal - do so at your own risk!! Since the options change occasionally, I haven't restricted the materials, but please consider it all carefully. Don't expect it to clip together well in ceramic for example!
Your best bet is white Strong & Flexible Plastic (which is also the cheapest!) and a can of gold spray-paint. I haven't tried one in the polished finish, I think that the detail would disappear. I've printed a small number myself as gifts: Some of the detail sometimes disappears in Alumide and I've also had one slightly dodgy WSF print but usually they are fine - either way I scratched the detail back in using a pin.
There is some more information available here: https://sites.google.com/site/ourenthusiasmsasham/creative/medallion
It is of course up to you to gild (cover with gold-leaf) your own. If you are lazy, I recommend it in WSF dyed yellow or orange (when available) - it is probably close enough for a fancy dress costume - though you could also try aluminide and a metallic spray paint.
I just added photos showing that it looks quite good in WSF in a gold-paint marker!