We did all first naps out in the buggy/car seat, as we had to go and see to my two horses. It was a godsend! Not only was I out and about every day, but we never got to the stage where DS refused to nap anywhere but at home.
A consistent time for a nap out of the house might be good. Even now at 7yo, DS likes to have a lively morning out somewhere and then the afternoon at home to chill out.