>>1745301a small trunk bag may work for you. ortlieb has a nice one (called the "trunk bag" because that's how ortlieb rolls) but is spendy, amazon has the usual aliexpress stuff with randomly generated brand names like sahoo and wotow. if you want to go even cheaper, diy with a small bin or bucket, bungee cord, and hooks.
a handlebar bag would be better as then you can access it while riding but idk if you have space for more than a very small one
