What I usually do when I take the brompton to paris in Eurostar, I have the front touring bag with all my stuff, then a carradice saddle bag in which I carry the bike carrier bag. Then I put the bike and the carradice saddle bag in the big carrier. I would have the big carrier in hold luggage and the front touring bag as a hand luggage. I could fold the cardboard and attach it to the saddle bag I suppose...
What I usually do when I take the brompton to paris in Eurostar, I have the front touring bag with all my stuff, then a carradice saddle bag in which I carry the bike carrier bag. Then I put the bike and the carradice saddle bag in the big carrier. I would have the big carrier in hold luggage and the front touring bag as a hand luggage. I could fold the cardboard and attach it to the saddle bag I suppose...