This question reminds us that lettuces don't like too much hot weather.
To look after my lettuce crop and guard against bolting, might it be worth covering the lettuce with, say, layers of damp newspaper for short spells during very hot weather?
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Some sort of makeshift shade would be a good idea; however, layers of damp newspaper could get blown away by a gust of wind, if they dry out and you forget to dampen them again. I use a 3m grow tunnel (high-density, fine mesh polythene netting), which provides the necessary shade and also protects them from birds and slugs.