I bought a big yellow dry bag at Walmart for $12 several years back, and it's still waterproof.
Dry bags are very tough, even the cheap ones. Get the one with the most convenient shape and arrangement of straps and closures.
My Wally World bag doesn't have any external straps, so I have to be extremely careful about securing it.
Get a bright yellow one -- it'll help brainless cage drivers see you better, and over time it will accumulate lots of filth that will instantly tell people you are a highly experienced and possibly dangerous world traveler... :biggrinjester:
2002 Suzuki DL1000 V-Strom, Dark Metallic Space Blue
1983 Suzuki GS850G, Cosmic Blue
2005 KLR685, Aztec Red - Turd II.2, the ReReTurdening