I use a 1"x2" from the local hardware store and cut 45 degree angles on two sides with my table saw. I glued it down and then covered them with dirt. 45 degrees makes doing the corners and such much easier if you make turns in the berms (see below). I'm sorry I don't have any "before" photos of the 1"x2"
Colorado Front Range Railroad: http://www.coloradofrontrangerr.com/
I would use cardboard covered with plaster soaked paper towels or guaze.
Consider cork roadbed (although it has the wrong 'angle of repose' even turned so the bevel is on the base) or similar material. It can be notched or cut if needed to bend to a tighter curve, then filled in before 'grassing' or other final appearance treatment.
Could use a concrete dike instead of a berm
https://inspectioneering.com/journal/2013-07-01/3513/tank-inspections-per-sti-sp001
I tried to sell my two cents worth, but no one would give me a plug nickel for it.
I don't have a leg to stand on.
I think I would cut the berm shape out of styrofoam, then scenic them for the look you want.
Mike.
My You Tube
What is the best way on making oil tank berms? I will be putting an oil refiner/petrochemical plant on my layout and need these containment berms.