Sindarin Translation for 'Golden Feather'

Would this be Glor + pess, making Glorbess via prestanneth?

If you are intending just to say "golden feather" generally, then pess 'lórin would be better. But if you wish to craft a name, then yes, Glórbess would be correct.

Glórbess would be correct

I'd say Glorbes(s), short o (cf. Glorfindel, Glorfinniel, Glornan) and optional single final s (cf. lass, loss > -las, -los, etc.). Consider also Glorphes(s) with possible liquid mutation, depending on when such a compound was formed.

"Would be correct" is a risky statement in practically any neo-Elvish undertaking, haha! None of us is Tolkien.