Stacker towers are great. However, they have a flaw. On one of my current mines, I have plans to dig it out as a pit, then fill it back in with rock- but in order to do so without collapsing the retaining walls, I will have to rely not on stackers, but on dumping. Why? Because while stacker tower extensions can be placed mostly-underground, stacker towers themselves can not, and I do not have a secondary excavation to place them inside. What I have is this:

(Yes, I know, that's the wrong distance from the excavation to get the extension footings to fall between the walls)

The total depth of that excavation is 25 units- or 26, from the elevated dirt the tower is projected onto. If the tower was just barely high enough to get over the uppermost retaining walls, however, the Tower could fill up to the top of the lowest set of retaining walls, and still have enough leeway in the 24-tile placement range of the extensions to continue placing across the entire expanse, enabling the use of stacker towers to fill in wall-retained excavations. That would also restrict the use of stacker towers to place tall piles of, say, depleted uranium (I have a friend that used them for that)- hence my suggestion: Multiple heights/"sizes" of stacker towers! They could all use the same extension, given its height flexibility, but to not be forced to excavate a place for the tower itself when I want it to fill in the bottom of an excavation or crater without piling all the way to the top...

In case you're curious what this large excavation is, it's an oceanbound bauxite mine that I plan to fully deplete and turn into my endgame oil refinery- but it'll be easier to make that refinery if the terrain is flat rather than excavated, nevermind getting the pipes in & out. Don't mind the extensive ocean reclamations I had to do to render about 30% of the deposit reachable- yes, I'm seriously being forced to mine the same rock at least twice, for a single operation. Maybe a dredging ship could help with that (the bauxite is found down to 20 tiles below sea level), but that's a different suggestion.