The roof base is a sheet of ½” MDO cut to 40” square. I cut a ½” square notch in 4 pieces of 2x4 which I had trimmed to 2 ½” by 1 ½” by 42”. I mitred the corners and joined these together with glue and screws.
I built a 6 ½” tall by 8” square box from plywood and small pieces to support the sloping walls and the light. Two pieces of MDF glued together, with a round hole in one for the light fitting and then trimmed to 8” square made the base of the beacon. The sloping roof was made by cutting MDO to the correct shape (a trapezoid 40” at the bottom, 8” at the top with a height of 16.28”), and then sloping the edges with an angled table saw cut so that they meet with a glueable surface.
The sloping edges sat on an indent (also trimmed with the table saw from a 2x4), which looks like this:
Much of the joining here is done with glue, as the roof is not really structural (unless you intend to climb on it, Adric style).
I ended up cutting a hole in the bottom of the box to fit a light to the interior of the box as well.
The beacon on the top is a jelly-jar light fitting. These are about $10-15 in the US and still used. I put a light fitting in it which is controlled through X10 so it will flash on command. For the rain hat above the light I cut an 8" square of bathroom board, and sat it on four sections of 1/2" brass rod spray-painted black. The rods have an 8" bolt going through each, which reaches down into the base. They're not actually screwed in place, but hold up very well by themselves.