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.)