No, there's no way to freeze your account .You have to pay at least once every 90 days or else you'll lose your number and account. But since your trip is less than 90 days, you don't have to worry too much.
After 90 days of $0 balance your number would be considered expired and would be lost, after which you would need to sign up for Public Mobile again under the new rate plans.
Being as you will only be away roughly 43 days you should be fine especially if you're keeping your balance active up until the time you depart.
