The large pieces of timber seemed to be an old bookcase. There was a few meters of cross beams. And the purchase of 3mm MDF as the surface. On top of the surface is 25mm polystyrene.
Thus for budgetary reasons, the base board has been great. I choose polystyrene for budgetary reasons too. If I had more money, I would have purchased 'expanded' polystyrene. Thus, the 'budget' polystyrene is meant to be more difficult to work with, and is meant to bead more easily. Time will tell if this decision causes me grief.
Initially, the legs for the board was going to be an old Trestle Table. Originally, I only had 2 cross beams. However, when I placed the frame on the Trestle Table, the frame wasn't as stable as I wanted. So I added 2 more cross beams.
The idea was that the top of the polystyrene sheet is at the same level as the top of the edge boards. I then glued polystyrene to the top of the 3mm MDF. I used normal construction adhesive. I have since learned to read the labels on the side of the adhesive. I now know that not all construction adhesives bond with polystyrene. I now use a 'fast bond' construction adhesive, because the label states that it will bond polystyrene - this brand has a 'normal' construction adhesive, but this does not bond polystyrene.
Just to placate other members of the household, I painted the polystyrene immediately. First I had to choose a colour. So out comes the old sample pots to test some colours.