A laser-printed , laminated "plan" of the field, showing "you are here" Public footpath is A STRAIGHT LINE between red and white striped posts at this end . alternative path at other end is A STRAIGHT LINE between the yellow /black striped posts.
YOU MUST NOT CROSS ANY PART OF THE FIELD BETWEEN THE 2 PATHS
Get some made for the red and white posts, ditto for the black/yellow (your colour choice may differ) fix to exterior grade ply screwed to the posts you could make the ply oversize and put hazard tape (cheap from Screwfix) round the borders.
A3 prints would be a good size, not too dear and a decent stationery outlet should be able to do them at a reasonable cost. Carry an aerosol-type foghorn (boat chandlers,maybe builder's merchants?) in the aeroplane. first pass, blast the feckers, next pass land whilst blasting them again.
I must admit the live fence-tape between the demarcation -posts ,has a great appeal !