Does your current 2.5 setup maintain a fairly stable water level? My only thought would be that structure set on top on the pond bottom could be moved if it turned out to be to low or to high for the regular water table.
If you do dig a hole to set them up, then I would just drill a couple of holes near the bottom of the 6" PVC, and stick a 1"X1' piece of PVC through it. The compacted soil should keep it from moving.
Just my 2�.
