I have in my collection the clip-on ammo box for MG3,. I see if I find it tomorrow.
Thank you, Hermann for the reference photos. So you’re suggesting that those patterned lines should be added on the box surface?
Depends upon if you can do it.
As thin as those lines are, they look like maybe a millimeter or two at most, it may not be worth the effort. The best way might be to try painting them, using a very fine brush with maybe one or two layers of paint, maybe three, then spray the color over them.
Ken
Eh, can’t do. Too small, too thin. 
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Cut narrow strips of decal film?
Like I said, it may not be worth the trouble.
Ken
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Get some Tamiya masking tape and cut super thin slices for your vertical lines.
MR Modellbau has a set of MG3 that includes two types of ammo boxes.