Watch tower.

I also would like to know. My guess is with concrete

Cheapest way would be like stone beams 8 stories up, some ladders then a floor on top to place the beacon on

Stone is the quickest tech wise. Its an annoying mission. You can also use diagonal beams to support colums but ive rarely got it to work right

I've built ones using only wood by using lots of beams and trapdoor floors. Each floor has 4 vertical, 4 horizontal and 4 diagonal beams. Using a stone foundation might make it easier to get more than 8 stories.

I use stacked Stone Foundations, with Stone Halfpieces snaking up around the outside. Using the halfpieces meant you could get ramps, corners, and half floors with a single crafted item that stacked so just had to grab two stacks of items on the way out the door. All you need to do is line the stone foundation under the hologram and then build to the correct height. Easiest way to do it I've found.

Dirt foundations have been added since, and may work as a substitute. Ladders may substitute for the halfpiece ramps around the outside of the foundations, but ladders tended to cause more problems than they solved in my experience.

Foundations are immune to storm damage so can easily remain as a stable tower without maintenance needed. I'd usually remove the ramp if I was leaving the tower behind as a result. But most of the time it wasn't a good location so I'd tear the whole thing down to use the materials for the next Build SMLP3 Quest.

I use wooden beams, wood floor pieces, roof and always criscross my beams for support. I then run a roof piece up each side attach a floor piece and keep going to the top. At the top I lay wood beams across the top and set the beaon on top and bam, done. The pod comes down with my reward.

My latest post of our build has a watch tower. We went full concrete but you could just use concrete posts and then put stone walls and roof on top if you want to go as high as possible as a cheaper option.

You could even do wood walls up if you like as the overhang on the roof protects it from storms.

The way I built mine on Olympus Open World was a 1x2 wooden platform suspended off the ground with 6 wooden poles, and stairs leading from the ground to one side. Go up the stairs, take a left, and build stairs going up, supported by wooden poles. Two more floors with accompanying stairs & poles, repeat until desired height. It's cheap and functional. Oddly, despite numerous storms the tower never disappears (it takes damage, but not 100%) in Open World - must be flashed as a Mission item.