I use this leather passport cover which I got from The Wallet
Shop. Good leh, I feel it keeps my passport in good condition,
easily identifiable when in groups, and important info can be kept
with the passport in the passport cover , i.e. international
driving licences. Inconvenience is when I go cross customs, need to
slip it out.
But yeah sometimes a bit bulky. On trips where load is important, I
just keep it in a ziploc bag or a waterproof pouch.