Summary

This was a huge project of mine, and probably isn't finished yet. I created a trench terrain that you can put on top of your table (and don't need to create an explicit trench board with the trenches dug into the ground). I feel like all approaches have their pros and cons, and I felt that "place on top" trench terrain offers a great flexibility, since you can use it with any table. If you don't like this approach (and the look of that), there are several other designers out there that offer other solutions. For now, you'll only get the place on top variant from me.

There are 22 different variants, with 8 different models each (at least on Cults3D. Since other platforms have quite some problems, I only added one variant each here, but you can download them all (for free!) on Cults3D: https://cults3d.com/en/3d-model/game/trench-terrain ).
I, T, X, L are the 4 basic types, representing a shape like the letter.
O and E are end pieces, open and closed, and OT being a T intersection with one open side.
HL, HI are just half elements of the L and I elements.
BrS and BrL are bridges (small and large). I also separated the bridges from the base for better printing.
B1 and B2 are bunkers.
The 8 missing parts are aimed at an open area, where you could place artillery.
BXT: X intersection with one open corner.
BT, BTM: T intersections with one open corner.
BS: Mostly open with just one trench corner.
BO, BC: Mostly open with two trench corners.
BL, BI: like the L and I pieces but with one open side.
BF: Just a flat open area.

For rendering, I added some other models:
https://cults3d.com/en/3d-model/game/interstellar-army-field-artillery-carriages
https://cults3d.com/en/3d-model/game/interstellar-army-field-artillery-weapons
https://cults3d.com/en/3d-model/game/rebel-guerrillas-army-for-grimdark-future-firefight