Sorry, but your last sentence made me laugh out loud. When I was a little girl (long ago, when dinosaurs walked the earth), we used elastic garters to keep our socks in place. At their most basic, nothing more than loops of 2 cm wide (well, 1", back then) elastic the diameter of one's leg, with the ends sewn together. That might work, too. But wellies are not ideal for long walks, so you may have to either split it in two and do half first thing and the other half in your lunch break, or accept you can't walk quite so far while it's so wet. As my mother said yesterday, we'll probably have a drought this spring/summer!