Results
NPR1 (rubber) - As noted in the initial prototype, the bowtie pattern corners twist when stretched. This torsion is transmitted across the entire upper member, causing the "top strut" (the vertical piece) to buckle "outwards". This is also apparent in this finer design.
hourglass21 (rubber) - One way to mitigate this effect is to make the "V" parts of the bowtie narrower and the strut wider. This reduces the amount of torsion the upper member can deliver and also makes the strut stiffer and less amenable to buckling.
klingon_5 (rubber) - Whereas the bowtie expands in the y when stretched in the x, the hexagonal cell designs are "radial". Whether stretched in the X or Y, material reacts by expanding in the orthogonal direction. Unfortunately, I was unable to create a curve that sufficiently "wrapped around" the central circle in the design. As a result, the triangles did not "unwrap" the circle enough to produce much of an effect, as demonstrated below.
Note that if these designs are rolled into a tube, the NPR effect becomes more apparent.