Lyle (Josh Brener) wears a "What The Fett?" t-shirt during the quidditch game. See screenshot at 00:58:00.
Mattress Salesman (Will Ferrell) wears a Boba Fett costume in an additional scene during the credit sequence. See screenshot at 02:05:00. Ferrell has several lines in costume, two of which mention Boba Fett exactly.