Originally posted by Nerevar:

the trophys is returned if you change it or move the ballista. its not lost. that would have been stupid. some trophys also target a bunch of foes.

Maybe I made a mistake, but it doesn't seem like the trophy is consumed at all. You simply use it to "attune" the device, it should stay in your inventory the whole time, unless I made the mistake of grabbing two trophies instead of one...

They also have doubled ammo capacity now. 40 black metal missiles to defend against nightly seeker stragglers is a fine deal by me.