The birdsmouth method in the link you suplied is as good as any. The only issue with that method versus a boxed mast is the difficulty in inducing a taper and with a 50' mast you want to defo have it tapered.
The difficulty lies in building the jigs to induce the taper.
Couple of tips:
1. Make sure your router table and extended platforms or rollers can accomodate the full length 50'+ pieces, or: depending on your router and skill set sometimes it is easier to build a table and jig that allows you to cut your joints with a taper with the 50' piece remaing stationary on an extended table and the router moves along a jig. This "table" can then be used later on as a clamping surface.
2. If you are making a birdsmouth column make up the 50' or so pieces with scarfs first so that they can be routed in one go for a tighter more precise joint
3. Plan your scarf joint locations in advance to ensure that no two verticle pieces have scarf joints are closer than 6 feet from each other
4. Clamping pressure is very important - too much or two little is not good. I see they used packing tape? Ummm, maybe that works well but i wouldnt do it myself.
6. They said it on the ducksworth webpage but I wanted to emphasize it again - DO NOT USE EPOXY
is known to catastrophically fail on wood joints that undergo rapid work cycles (flexing) and a mast is exactly that. The epoxy can be good for years and then with no warning suddenly let go.
7. If you are building a boom as well build it first using the same technique. It is a great way to test out technique as If it goes pear shape it is less time and money lost
and you can go again before you commit to the stick itself
You are looking to build a 50' mast, I believe the link you supplied was a 27' (9meter) mast. It is WAY more difficult to do a larger mast, but very doable. It will cost you more than you think however and usually any cost savings from building your own mast arent acheived unless you build a couple...