Whittle the dowel or straight stick into a gradual point. The end should be a conical shape.
Blunt the sharp end of the stick by cutting off the point and rounding it. Leaving the end sharp can be dangerous when you insert the stick into the air mattress.
Wrap a few layers of duct tape around the stick.
Test the plug by firmly inserting it into the hole. The wooden plug should go in smoothly and provide a tight fit. If it doesn't, remove the duct tape and shape it so that it fits snugly.
Inflate the air mattress and quickly insert the plug.
Put duct tape over the end of the plug on the outside of the air mattress to prevent the plug from popping out.