Adding an overlay material to a zoom function?

I’m making a rifle scripted weapon, and I have added a zoom function. However, I’m not sure how to add an overlay material to it (for the scope)

Here’s the zoom code:

[LUA]
local zoomed = false
function SWEP:SecondaryAttack()
if zoomed == false then
zoomed = true
self.Weapon:GetOwner():SetFOV(2,0.5)
self.Primary.Cone = 0.5
else
zoomed = false
self.Weapon:GetOwner():SetFOV(90,0.5)
self.Primary.Cone = 0.15
end
end
[/LUA]

Network the zoomed variable to client and draw the thing.