[G6Tanuki]

Get up as much wire as you can! Then end-feed it against a good earth (lots of buried wires, and/or anything else metal)

One trick worth trying - to help make the wire look more like a 1/4-wave (so you get a healthy RF current flowing in the outdoor radiating bit rather than merely circulating in the ATU) is to bring the far end down the support and then zig-zag-load it: the zig-zag wires only need to be separated by a few inches. Essentially you're 'folding up' the far end of the wire (the bit which carries little current so contributes little to the actual radiation).