In a medium bowl or a half gallon mason jar, toss together cucumbers, onions, peppers, and salt and set aside.
Combine vinegar, sugar, mustard seed, celery seed, and pepper flakes in a saucepan and bring to a boil. Once at a boil, remove from heat and add 2 handfuls of ice.
Once ice has melted, pour vinegar mixture over cucumbers. Cover and store in refrigerator. Will keep for 2 months.