A Royal Guard, eh? Interesting choice. How sturdy do you want this costume? Is it something you plan to wear once for Halloween or to a convention, or is this something you plan to join the 501st with? If it's the latter, I'd recommend just buying the helmet from one of the various helmet makers, which you can find more information about on the Sovereign Protectors forum (http://www.forcepike.net/)
. This wont be cheap, nor will this costume. The material will also be very expensive.
I know a couple Royal Guard costumers and they've both purchased the helmet, as well as whatever else they could get pre-made (which is pretty much everything but the soft goods). But this is for the 501st.
If you're just looking for this as a one-time-only costume you could probably whip up the helmet with cardboard, duct tape and spray paint. Use cheaper red material (and wine-colored material for the pants, shirt and gloves), and there you go. Rubies also made a Royal Guard helmet a few years ago but I have no idea what the price for one now is.