I would use 18-20 gauge mil spec silver plated copper with teflon insulation. If you want something more special than that, use the same mil spec wire, but get it from someone who cryo treats it, still should be relatively affordable. I do not like to use any wire which is bare copper, because it corrodes in seconds in an oxygen environment, and even insulted copper wire tends to corrode fairly quickly under the insulation! The silver plate protects the copper (silver will oxidize as well, but silver oxide still conducts well, unlike copper oxide), solders really well, and makes great connections with pressure as well (crimps and screw terminals). Alternatively I would use Cardas litz, but I generally reserve that for AC use.
Likely your biggest gains will come from making the umbilical as short as possible, DC does not like to travel very far, and twist the +/-(G) leads for good measure against noise pickup.
