I just popped in to Decathlon and bought two of the proper tubes. What I will do now though, is maybe carry the 700x32/47 tube as a spare since it's smaller and lighter and I'd normally just patch on the trail/road anyway so it'd need to be a big failure before I put another tube in.
Any reason not to use a 700x32-47 tube in a 29x2" tyre?
Same diameter, right? Just the width of the 700C tube is more like 1.75" than 2" so it'll be more stretched.