I’ve just bought one of the light weight models from DD Tarps. It’s about 3m square. I gave it a quick try on an activation on Saturday but the ground was so rough I just wrapped it around myself with the rucksack between my legs and the radio inside the rucksack. It wasn’t elegant but it kept the showers away.
There’s a thing called the Zelter Shelter which looks interesting. But at 1.3kg it’s a bit heavy and it isn’t cheap either. It’s inflatable and could be used without pegs if you were on rock.