Do you ever wish you had a durable and waterproof pannier? Do you also wish it would cost a handful of change (rather than a couple hundred bucks)? Meet the bike bucket:
(photo from 2005, my bike has changed a bit)
I was using extra bungie straps in the early 2000s because I was a student and would haul huge loads of books with me daily. Two straps made the bucket (& me) feel more secure.
I have a 3-4 of these buckets, the total cost was about $5 since I had extra buckets and bungie cords already. If you need square or rectangular buckets ask friends with cats (cat litter comes in these sometimes), restaurants, or grocery store bakeries (sometimes food comes in these, like mayo or cake icing). These buckets are great for hauling just about anything, keep the contents 100% dry, and will even survive a bicycle wreck without spilling the contents (guess how I know?). I still use them, but they aren't a permanent fixture on my bike like they used to be.
A big thanks to my BFF, Jody, for pointing this tutorial out to me 10+ years ago.