Sometimes with a strip each side of angled metal to minimise bending caused by the constant pressure of the leaning enlarger head.
The OP mention he had a Beseler 23CII-XL and I looked at my Beseler 23CII-XL, which had no such metal and is 3/4inch thick. Nor does my very large Durst 4x5 although it is 1.25 in thick. However, on my Beseler 4x5, the whole frame is attached to angle iron which then attaches to either (3/4 inch thick) wood or MSD, depending on the vintage of the enlarger.
I've seen some particle board baseboards warp over time. Plywood may be more rigid. Rather than using really thick plywood, consider two layers of relatively thin plywood separated by a properly designed and glued pattern of wood strips to function much like an I-beam. It can be stiffer than a solid baseboard of equal weight, but thicker.
If the base of the column has a small area of contact with the baseboard, it can compress the wood fibers and cause the column to tilt slightly. A metal plate to distribute the pressure can reduce this.