I have continued the work on the underside of the layout. I started by flipping the benchwork so that is was resting on its back. That made it easier for me to work on the underside. The first thing I did was to paint the underside. I simply rolled it twice with white latex paint. The kind you normally use for walls or ceilings.
Next I wanted a way to be able to easily fold down the "hatches", in order to be able to access the layout from in under. I decided to use a piece of threaded steel rod and a wing-nut to hold each hatch in its up position. Unscrewing the wing-nut would loosen the front side of the hatch and it would swing down.
I begun by cutting a suitable length of threaded rod.
Next I drilled a hole in the front benchwork girder. I made the hole slightly narrower than the rod and forced the rod into the hole with a hammer. That seems to be enough to hold the rod in place, even under the weight of the MDF board hatch.
A hole in the hatch, a washer and a wing-nut completes the whole thing.
Last is a picture of the benchwork when flipped back in its correct position. One of the hatches is down and the others are up. A nice and tidy underside, suitable in a room also used by the rest of the family.