Yay another GGO cosplayer!
I am trying to construct Death Gun's L115a3 assassin rifle, so I am in the same boat as you. I blew up some reference images and created a template on card stock and plan to cut the body out of some spare 2x6 lumber I have laying around. For the barrel and suppressor, I am undecided between using gas pipes or PVC. The scope and details have yet to be worked out ._.
However, the hectate II looks a bit more complicated to create since the stock and the body look like two separate pieces. Also doesn't look like you can have a fall back plan of just buying an airsoft replica like I can. I hope someone who has actually created a rifle can chime in and provide some advice!