Cryogenic flexible printed circuit ribbons based on stripline transmission lines have been custom designed, fabricated, and characterized for an upcoming NASA Class-D balloon mission. The Galactic/Extragalactic Ultra-Long Duration Balloon (ULDB) Spectroscopic Terahertz Observatory (GUSTO) will make use of 8-channel flex ribbons to transmit Intermediate Frequency (IF) from 0.3-4 GHz while at cryogenic temperatures and in flight. In this configuration one individual flexible ribbon can replace up to 8 semi-rigid SS-SS coaxial cables that are 20 inches in length. The GUSTO focal plane array consists of 24 pixels that would otherwise require an equal number of coaxial cables. Similar performance can be achieved using only 3 sets of flex ribbons, each of which is comprised of two circuits in series. This will not only reduce the weight and form factor of the IF harness, but also provide improved thermal performance. The ground planes of the flex circuit have been reduced to minimize heat transfer into a cryogenic system which can extend mission lifetime by mitigating coolant boil off. Each channel of the flex circuit conducts a quarter of the heat that a coaxial cable would. To accommodate the unique design of the GUSTO cryostat three pairs of flex ribbons will be RF coupled to provide the necessary length and shape for routing between IF components. Flex circuits were designed with a staggered array of SMP type press on connectors instead of SMA which makes coupling 8 channels achievable in one easy maneuver. The circuits can be clamped together to prevent separation in flight or other high vibration conditions all in less time than it would take to install coax. All RF testing was done using a closed cycle vacuum cryogenic station with the test bed held at 20 K. Each pair of flex ribbons was thermally coupled to the 20 K stage. The flexible transmission lines showed an average insertion loss of 3.07 dB/ft at 5 GHz, while the industry standard SS-SS UT-85 coax loss is 2.80 dB/ft. The GUSTO IF system will use state of the art ASU cryogenic low noise amplifiers to more than make up for the higher insertion loss. Any future systems that can correct for, or accept the additional loss will benefit more from the other features of the flex circuit design. The custom made cryogenic flexible transmission lines are still the preferred method of signal transmission for applications that require complex routing, lightweight components with small form factor, easy assembly, lower heat transfer, and flexibility.