I was just reading about the Walwin grasstrack frames of the 70's - aluminum plate bolted together sandwich <10 lbs sans swingarm (tubular steel). Yetmans are around 8 in chromoly.
Victor Pagan had/has a Bidalot RS1 replica frame done up in aluminum - you could look at a machine like that or the peugeot xg3 for ballpark geometry and scale.
a single plane box section seems to do the trick too. Foale made some cool spine frames.