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Are there any official (or well written fanmade) rules regarding trench warfare for 7th?

I've seen a few WWI/Trench warfare/Iron Warrior hard-on boards, and IIRC, they suggested simply using the Aegis defence line rules for cover. If the trenches are muddy enough treat them as difficult terrain.

If you can model it that is fantastic, what I would recommend is treating the trench/wall as a "fortification" for a 3+ cover save. If you actually make the trenches you could go a step a further and block LOS for small models inside, though you'd want to make sure both sides have barrage weapons if you do that.

I'd put the closest trenches about 20" apart, with some craters (5+) and difficult terrain. That way an infantry charge takes 2 turns, but there's no question that most basic infantry weapons can still shoot across the gap. Jump units could make that charge in one turn, but they'd need at least an 10" charge roll to connect with the guys on the other side (12" move, -2" on the charge for terrain). You can adjust that distance depending on how effective you want assault to be. Have more fortifications behind for artillery, extra troops, etc.