I had seen such a schematic years ago. Drawing a mental picture, from
what
I can remember, a wire ran down the center of a tube on a couple of
insulating spacers (washers) in a concentric configuration.

A small hole in each end of the tubing allowed an insulated pickup turn to
be run adjacent to the wire in the tube--exiting the holes in the sides of
the tubing, this picked up sufficient reflected power to drive the led(s)
and give an indication of SWR (led(s) dim or out at 1:1, or near.

Anyone got that schematic/construction details or, one of a similar unit
to
construct?
========================
As an alternative to the above you might consider a piece of 'air-spaced'
coax (10 mm OD) as used for sat TV ,say 100 mm long ,connecting the centre
conductor to the sensor's in- & output connectors, earthing 1 side of screen
only.
By inserting a (possibly teflon) insulated wire through one of the 'air
gaps' you create a suitable pick-off for forward/reflected power (SWR)
measurement. The 75 Ohms impedance of the short piece of coax is of no
concern in a 50 Ohms environment.

Frank GM0CSZ / KN6WH