i don't know what you exactly try to achieve but if you wan't to have the plane center at the top left position of the screen
local pos = self.camera:UnProject(Vec3(0, 0, 1))
-- always face the camera
self.entity:SetPosition(pos)
self.entity:SetRotation(self.camera:GetRotation())
self.entity:Turn(270, 0, 0)
this works for me.