The experimental plastic base of the sea wall looks OK but I've yet to paint, drybrush or flock it so I'll hold off judgement until I do that. The hardest part was finding a glue that sticks wood to plastic!
Meanwhile, I've found a great solution for the anti-tank ditch. We need a flexible material which will sit on the sloped hills of the hex terrain. We were thinking of just masking tape but I had a brainwave and have used some non-slip dashboard mat instead. This can be cut into thin strips and painted with different shades of brown (dark in the middle, light on the outside) which gives it a 3D effect of a trench. As its flexible it will fit along the contours of the hills and will (hopefully) look better than just masking tape.
As ever, photos when a) the bits are finished and b) I can find the cable for the digital camera!