I have found that SU-8 photoresist at 50-100 microns holds up well to rie
etch of extended times. How deep are you wanting to etch? The problem with
SU-8 is getting it off after etch. It will ash off but it takes a while.  Bob

Gold might be an acceptable mask for you as well but remember that gold will
rie sputter an coat your chamber. If this is not a problem then it is fine.
Bob