Is there such a thing- or should I try to build one.

There is an old 10x10 cistern in my basement. It is under a slab,
but two walls are exposed. The soil is clay. I've cut a door in it
and drained it, but there is still seepage from somewhere. I want to
trench 4-6" channels along the buried walls and fill with stone.

I haven't seen anything that long, but I'm thinking I could fabricate
a ditchwitch style trencher that ran off a 1HP electric motor geared
down. I don't care if it takes a month to dig one side.

Has anyone seen a long trencher- or plans to build one? Or have a
better idea on how to drain the clay before it gets to the cistern
walls?

thanks,,
Jim