Rashi on Exodus
צנצנת is an earthern flask, as it is translated in the Targum (cf. Mekhilta d'Rabbi Yishmael 16:33).
Rashbam on Exodus
ויאמר משה אל אהרן, Moses said this to Aaron only after the Sanctuary had been built and inaugurated, i.e. at least about 11 months later. Until then there had not been a Holy Ark.