I live in Michigan, we are not blessed with the longest growing season known to man. So we try to stretch that, and to stretch it we need to be able to protect plants from late frosts or Freezes as late as late May if we’re really unlucky.
You can put in a hoophouse or something like that but plastic sheeting does not provide a whole lot of protection.
But you can very easily build a styrofoam box to put over your seedlings. Think of it like a rectangular “rose cone” those styrofoam cones us northern gardeners place over our roses to protect them in winter.
Expanded Polystyrene Insultation (aka styrofoam) is a cheap building product used to insulate primarily basements. It comes in standard lengths (8 feet) and standard widths (14.5 inches, which is the gap between standard 16″ on center framing). It is also really really cheap. Extruded Polystyrene Insulation is more expensive (this is the pink board or blue board you see at the store) and works as a better insulator, if you need more protection. It also comes in bigger sheets which may require cutting (a utility knife works).
You’ll need lengths of the expanded insulation mentioned above to cover 1 8 foot row of seedlings. Stand two up on their sides, place a third on top to form a box, and using your hands press finishing nails through the top into the sides to secure them together. There are also glues you can buy if you want more security. Then duct tape the seams for added resistance and put ends cut to size on the same way (with additional duct tape for security) and you’re good to go. Just remember this thing is extremely light, so weight it down so it doesn’t blow away.
Now you can protect a whole row of seedlings from a late frost.