How do I get my baby used to sleeping swaddled with arms out?

My almost 4 month old son has been rolling over from back to belly for 2 weeks now during play. He sleeps in a snoo at night so I don’t have to worry about him doing it in his sleep typically, but we are going on vacation next week. He will not sleep without being swaddled and I’ve tried to swaddle him with arms out and have him nap in the travel bassinet to get ready for vacation, but he wakes up immediately because he starts thrashing his arms around. When completely swaddled arms in, he falls right asleep. Can we safely swaddle him fully while on vacation so he will sleep in the travel bassinet?

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