I originally wrote:
Consider using an UNBALANCED, open-wire line. It would consist
of three wires fairly close together with uniform spacing
between them. The center wire is "hot" and the outer two
are "ground".

Cecil, W5DXP asks:
Hi Jim, what would be the range of Z0's possible with this
configuration?

Jasik has a formula for this type of line:

Zo = (207/sqrt(epsilon))*(log(1.59*(Dist/dia)))

where Zo is in ohms, epsilon is relative dielectric constant
of surroundings (=1.0 in this case), sqrt is the square root
function, log is the base-10 logarithm function, Dist is the
distance between the center wire and one of the outside wires
and dia is the diameter of the wires used, Dist and dia being
in the same units.

Setting dia=0.116 inches (#10 AWG stranded wire) and varying
Dist gives:

Dist = 1.0 inch ............ Zo = 235 ohms
Dist = 2.0 inches .......... Zo = 297 ohms
Dist = 4 inches .......... Zo = 360 ohms
Dist = 10 inches .......... Zo = 442 ohms

The trade-off would be between the desireable high impedance
from wide spacing and leakage suppression from close spacing.
Dist=4 inches looks about right.

Although I haven't tried it, my guess is that common electric-
fence hardware would be adequate for supports and insulation,
but the conductors should be copper (and #10 gauge). Stacking
the conductors vertically should simplify the mechanical aspects
and possibly partially suppress sky-wave leakage from the line.

Jim, K7JEB
Glendale, AZ