There seems to be some confusion over the purpose of the sprung legs. Some people theorize that the legs "couple" the magnet to the surrounding soil. While the legs may provide some "coupling" effect, their true purpose is to hold the magnet in place while the borehole is grouted. After that, we don't care about the spring force of the legs or their resistance to corrosion.
The soil mass moves up or down and carries the magnet (and the grout and the pipe) along with it. We recommend using a soft grout, but unless the borehole has a large diameter, there is little chance that the grouted hole will become a pile, regardless of its strength.
We have strapped magnets directly to inclinometer casing with telescoping couplings, grouted the hole, and seen perfectly good settlement measurements - without the use of spider magnets at all.