I had a similar goal in mind, but that also included being able to place a bonfire at the top. This meant I got a sufficiently tall beacon tower, if not "reaaaaaaallllly high" ;)
https://steamcommunity.com/sharedfiles/filedetails/?id=2434814390
The height of any structure is limited by the distance from the natural terrain it is built on. So, in order to build higher, the trick is to raise the terrain as high as possible within the structure in order to raise the height limit ;)
It's not Barad-Dur, but it means that my puny initial attempt...
https://steamcommunity.com/sharedfiles/filedetails/?id=2433938251
... looked a lot better and worked for me once I raised it using the technique above ;)
https://steamcommunity.com/sharedfiles/filedetails/?id=2435699869
This works for me as I can see this from miles away and can easily navigate the Deep North waters and find my safe harbour without having to constantly navigate via the mini-map ;)