HDPE Threaded Plugs
Threaded plugs are an excellent way of ensuring that your threads are protected. They also almost eliminate any risk of the plugs falling out during transport.
Typically, you'll find specific protective plugs based on thread type (JIC, SAE, Metric), and then you'll want to determine what installation/removal method you want your line to go with. Whether it's by hand tightening/unscrewing or by using a tool, you may want to work with your lineworkers to determine their preference.