How do you keep veg seeds in pots outside in May when weather is iffy?

Just cover with plastic bags with a few holes so they can breathe. I use bamboo skewers stuck into the pot and place the bag over and fasten.

You can use empty milk jugs or large plastic jar-like containers from restaurant foods to cover the plants. You could also make a wooden frame and staple clear plastic over it and place over the pots with wood supports. Here is a list of suggestions. You can find a lot more by searching for "potted plant protection from frost." https://dengarden.com/gardening/Protecting-Plants-from-Spring-Frosts

You can set them under a patio table or deck to protect them on iffy nights when it may frost. You can also move them indoors. I have moved so many that I finally broke down and bought a cover for the plants at my local nursery. It is light weight (lighter than using the sheets I had used previously) and much better keeping old Jack Frost away. They are inexpensive and should last a life time.

Never plant outside til after Mother's Day last ground freeze; you'll need to protect them by covering or moving in or at least under porch