Can't go wrong with a old unsuspended MTB and slick tires. They are cheap and easily found. And the basic platform can be taken in lots of different design directions.
Decent frame, decent tires, maybe single chainring up front, and you can bring it in under 25 pounds easily.
jim
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Cross Check Nexus7, IRO Mark V, Trek 620 Nexus7, Karate Monkey half fat, IRO Model 19 fixed, Amp Research B3, Surly 1x1 half fat fixed, and more...
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SB forever