The tower needs to be beefy. I had some 7/8 ASH laying around so I used it. Everything hinges on the tower so don’t be shy adding a few bolts.
The lower part ( 2 1/2×15 1/2) is just bolted to the base with three lag bolts. I drilled the base so the bolt heads sat flush with the bottom. There are dados for the vertical pieces. I wanted to make sure I could minimize any movement and I think this works with the triangle supports. By the way the dimension of 13” on the tower includes the dado. The vertical pieces are 3/8” less, I just wanted all of the measurements to be from the base. The vertical pieces are glued and screwed into the dado.
The arms are attached by drilling an over sized holes through the tower at the correct height 3” and 8 ”. Here is where an improvement can happen. I had to make sure I didn’t over tighten the bolts or I would squeeze the arms solid. They need to move freely. The distance between the vertical pieces was made to fit the arms perfectly. No room for a washer or nut or ….. I think if I did it again I would try to put in a sleeve bushing? Anyway before putting the bolts in I waxed them up and did the same on the wood. It works, so far.
You will notice too that the bolt for the table is at 5” you need to make sure you give plenty of room for the lower arm. I used two bolts staggered above the table bolt.
The triangle pieces are screwed into place.