Solution:
Lattice currently does not offer an example flex cable solution to connect the Lattice Nanovesta sensor to the Lattice HDR-60 evaluation board. If you would like to design such a cable, the sensors will typically expect 50 ohm traces both on the PCBs and over the flex cable.
A reasonable flex cable design approach could be to have a bottom layer ground plane, and the top layer would then include all the sensor signals. The powers could also go on the bottom layer. It should not be necessary to include ground guard traces around the sensor signal traces. Differential signal traces should use a 100 ohm differentially coupled construction. If possible single ended signals should be separated as much as is reasonable to lower SSO.