For semi-29" tires like the 42's, I like plain 26x1.5-2.3" tubes. Stretched to 28" they are already thinner than they originally were and form a good match to the tire, so little tension. Just pay attention though, this week I inflated a loose fitting wire 45mm tire (all messed up shaped) with such a tube and it managed to get under the bead, so BOOM. Same would have happened with a 700x40mm tube I'm sure though. The tire was really messed up, been in a warehouse for too long before I bought it, and being shipped folded over.