Simon,
The 'shape' of a general purpose SWL antenna that will
be used on a number of bands isn't all that critical. It
won't be particularly 'resonant' on any band and it's
directional properties will be random, rather than something
easily predictable and 'aimable'. In general (meaning that
there are always exceptions), the longer and higher you can
get it the better it will work.
The 'folding it back on it's self' that you talk about
refers to the angle between the 'folds'. The smaller that
angle, the less effective length there is, to a point, sort
of. At the extreme, wrapping the wire back around it's self
effectively cuts the wire off at that point, again, 'sort of'.
Given the amount of area you have to play with, you have
several choices as to 'shape' of the antenna. Rectangular,
triangular, and almost anything inbetween, not to mention
in the vertical direction. If it were me, I'd try for what
ever 'shape' that would result in the most length. Since there
isn't ''one'' perfect length and height for an SWL antenna, and
since it won't be optimum for all bands, give one a try. Later
try another 'shape'. Keep the one that works best for your
location and band(s) of interest.
'Doc