Rain covers for all Bihn backpacks sound like a great idea.
Original designs. Made in the U.S.A.
Alosaks...Navy Seal approved...will tote most things and are water proof. Available in various sizes at Amazon. If we are talking standing in a downpour....Alosaks and/or a travel umbrella (the latter of which will not be utile while riding a bike).
The stockpile keeps growing...I'm in serious trouble.