Great, I'm just 2-3 km upstream from there. We could meet at 9am at km 17 mark (under the Kohoku bashi bridge 江北橋). About the bag, if you can't buy/borrow one by Tue, something from 100 yen shop might do. I know they sell velcro tapes of 2m, which you can cut to the size you need, and this is great for packing the bike. For covering it though, try some rain bike cover, or those spreadsheets for picnics/ hanami parties, even black garbage plastic bags have been used I understand (held by sellotape or something).I might also mention that I start my rides close to Shimachu Homes / K's Denki on the Arakawa.