Just realized I did not pay off on "how."
Would start with a vague rectangle for the base.
Would make a bit of a diagonal across that.
Behind I'd lay some 3/4" foam (blue open cell, not the spendy pink closed cell).
This would create the step "up" from the bottom of the ditch to the behind of it. This would really be 4-5 feet, but for dio purposed I think we could "get away' with only 2 scale feet.
Behind the diagonal, I'd lay in a 3' wide band to follow the diagonal. this wuld get an 1 1/2" wide piece avove it. That would get you 4 scale feet to set the H35 behind.
That placement would let a person set the final rectangle for the base.
The foam would need to have it's "ditchward" edges cut at a 45º angle (ok, so the natural maximum repse f grass is only 40º, I'm not taking a protractor to a dio when I'm cheating depths anyway. Beveling the edges will help you not spend as much on the base material--cell-u-clay, spackle, plaster, etc,
Do remember, though, not to put trees between the berm and the ditch--the ditch only really works if you can see down its length. (Althugh artistic license might allow for a couple.)