To draw out excess liquid and increase crunch, toss cucumbers and onion with salt in a large colander. Add ice, and toss again. Place over a bowl, and refrigerate, tossing occasionally, for 3 hours. Drain. Rinse well, and drain again.
Bring vinegar, sugar, mustard and celery seeds, peppercorns, turmeric, and red-pepper flakes to a boil in a saucepan. Add cucumbers and onion. Return to a boil (if using squashes, add them, then remove from heat; do not return to a boil).
Ladle into 3 hot sterilized pint jars, leaving about 1/2 inch below each jar's neck. Wipe rims of jars with a clean, damp cloth; cover tightly with sterilized lids and screw tops. Using tongs or a jar clamp, transfer jars to a rack in a large canning pot or a large, deep pot filled with hot water, being sure to keep jars upright at all times. (Jars should be spaced 1 inch apart, and should not touch sides of pot.) Cover with water by 1 inch. Cover pot, and bring to a boil. Process jars in boiling water for 10 minutes. Let cool. Press down on each lid. If lid pops back, it's not sealed; refrigerate unsealed jars immediately, and use within 1 month. Sealed jars can be stored in a cool, dark place for up to 1 year.