To make the chili sauce, combine the mayonnaise, chili sauce, chili powder, and cayenne in a small bowl and stir well. Cover and chill until serving time.
For the onion rings:
Slice the onions into very thin rings and separate the rings. Spread them out and sprinkle with seasoned salt. Place the flour in a re-sealable plastic bag. Put the onion rings, in batches, in the bag, close tightly, and shake until the onion rings are coated.
Pour the oil into a Dutch oven. Fry the onion rings in batches for 3 to 4 minutes, until they are brown and crispy. Remove with tongs and drain on paper towels.
Taste and season with salt, if necessary. Serve with chili sauce.