Photo from the Sukkot Exhibit, curtesy of Neot Kedumim Park
(ג) הָעוֹשֶׂה סֻכָּתוֹ בְּרֹאשׁ הָעֲגָלָה אוֹ בְּרֹאשׁ הַסְּפִינָה, כְּשֵׁרָה, וְעוֹלִין לָהּ בְּיוֹם טוֹב.
(3) In the case of one who establishes his sukka at the top of the wagon or at the top of the ship, although it is portable it is fit, as it is sufficient for a sukka to be a temporary residence. And one may ascend and enter it even on the first Festival day.