The probe holders in this folder are meant to do just that: hold in place water/soil sensor probes of various kinds. We typically use Atlas Scientific probes, but other such as DF-robot have similar shapes and could be adapted.
We normally use ABS or PVC plumbing pipes with some custom 3D printed parts in ASA or ABS. Then, we use some 316 stainless steel nuts and screws to keep everything together. Check each folder in case you are looking for a single or multiple probes.