What about this?

Keep the distance and timing finish line at the airfield (with a
minimum altitude of, say 30 feet in the UK, for go-arounds). Many
contests have a control TP close to the finish to line finishers up.
Just give that control waypoint a suitable minimum altitude below
which the glider increasingly penalised by points, and an absolute
minumum below which it is considered unsafe to try to stretch the
glide to the finish and the glider is scored as uncontrolled for that
point and so gets distance points only for the flight. Gliders that
reach the final control point below the lower minumum will have an
extra disincentive to carry on to the airfield as they will lose (not
gain) distance points by their scoring distance being radiused back
along the last leg from the uncontrolled final waypoint as per normal
scoring practice.

A suitable choice of control point position (as regards distance,
finishing direction and, crucially, a safe landing field) and minimum
turning height (for energy surplus for a safe finish) would ensure
that the fun for the pilot and spectacle for helpers and spectators of
airfield finishes is maintained. The control point position and
minimum height can easily be chosen so as to encourage either go-
around or straight in finishes as desired by the contest organisers.

[My preference would be to encourage fast finishes to a safe low
minimum height and crossing a line and not a cylinder at the
airfield. The logic being that, with the above regime, successfully
finishing gliders will flying at similar (and adequate) speeds and
glide slopes and the dangerous conflicts between gliders final gliding
at different speeds and heights and flying over and under each other
is minimised. Using a line they can spread out laterally without
penalty - with a cylinder everyone aims for the same closest point.
Having a low but sensible minimum altitude is safer than high fast
finishes which tend to lead to gliders flying over and under each
other because of different eyeball judgements about their height.
Most people can make a reasonable estimate of 30 or 50 feet.]

John Galloway