Not to contradict David Th, but if you are concerned about weight and want the top to remain light, you also have the option of inserting a somewhat heavier plug into the bottom of your tube instead, and extending it a few inches. If the tube is already tapered, I would personally prefer the extended heel plug, which, by the way, would also be much easier to make. This is because the walls of the tube will be parallel at the bottom, easy to fit a plain round plug. But if the lamp pole is tapered, fitting an extension to the top is quite... well...  I don't even know you would do it.

I agree with David Th, also from unhappy experience - it is better to have the mast too long than too short. If you need to keep your options open for retrospective changes, the bottom of the mast is easier to modify than the top, especially after you have made your masthead fittings. This, too, points to a wooden heel plug extension rather than a masthead extension.

(The beauty of a wooden top section is when you just have a parallel-sided tube. The plug-extension is then easy to fit to the top, and you have the advantage of being able to put some taper into the top of the mast.)

My lamp pole arrived. For some reason the 25' pole I ordered is actually only 24' 8" long. Not sure what is going on there. I need to measure all the other dimensions to see if anything else is off. I have no idea how I could verify the alloy and the temper. I suppose I have already decided to just trust the vendor on that.

At some point I determined that 24' was the absolutely minimum length for my mast. Hopefully that was correct.

I am able to lift it and carry it around. It is not easy but I can do it.

Pete was making tabernacles for larger, heavier, stiffer, ocean-going boats. I wouldn't use more than 3/8" studding on a 22ft boat intended for lake sailing. If set in epoxy, with large washers under the nuts, there can be no doubt about the overall strength of the assembly.

I have started thinking more about tabernacle details. I am following, as much as I am able, what Pete Hill wrote in JRA Magazine issue 61.

The only concern I have right now thinking through the build is the size of the threaded rod that I will need to glue into the sides of the tabernacle.

Pete recommended 12mm threaded rod for a sail approximately the size of mine. This works out to just about 1/2" in the freedom units we use here.

I am concerned about drilling 1/2" holes in four different places on the sides of the tabernacle. The boards I used are 2" dimensional lumber which is 1.5" actual thickness.

I am trying to decide if drilling out a third of the thickness will significantly weaken the structure.

Does anyone have an opinion to share?

Even high quality sanded plywood can have a tolerance of +/- 0.2mm to 0.4mm (or 0.008" to 0.016") and your plywood is well within that, at only 0.006" from the nominal thickness.