I understand the fear! Leaks are a pain.

Our veggie filters are EDPM with compression around a toilet flange on
the bottom. The inflow is from a pipe above the level of the filter
of from an adjacent filter through a sipillway.

Link to pic of 4 x 4 installation (and pond album):
http://picasaweb.google.com/jimandph...58152477668738

Jim