A very simple form of a piston attenuator for the OP's application is
nothing more than a metallic tube into which you can suspend the
receiver on a string. Adjust how far you drop the receiver into the
tube (held vertically) to adjust the attenuation. Seriously. You can
wrap a cardboard tube about three or four inches diameter with aluminum
foil to make the "meatllic tube." It's a trick that many locals here
have used effectively in hunting hidden transmitters.

A more usual form is a tube with a couple of coils in it, with a means
to adjust the separation of the coils. See the third paragraph of
http://www.measurement.gov.au/index....1D#attenuators
for an example.
http://ej.iop.org/links/q55/Y67w5gpi...17i12p1172.pdf is
an article about correction of small errors in (precison) piston
attenuators. This pdf has a cross-sectional diagram in it, but don't
get bogged down in the details.

Cheers,
Tom