I had heard that people use dental floss for mending tubular tires, because it is strong and comes pre-waxed. Mouth hygiene is important when doing long days on the bike, so, if you are already carrying dental floss, the only other thing you need to effect a temporary repair on a big gash in your side-wall is a needle. They don't weigh much, and a few Frankenstein stitches can stop the sidewall bowing out, which a boot wont do.
I did the repair below and it seems to be holding up well. The white (rim) tape is stuck on with some F2, but in the field I reckon you could get away with just a tube patch to stop the dental floss thread rubbing on the tube, (assuming you are now using a tube!). You should then be able to ride into a town where you can buy a new tire.