hello from europe. Seen somebody ask for a hard(suit)case for a Brompton. DELSEY's 4*4 fits a Brompton with rear rack and the standard seat(you don't even need to take it off). Think it is a french brand . Hope you can find one in the US. Seen it for 65 euros.
Globe trotter suitcase fits a brompton without rear rack,detached seat.185euros.
Thing is... they do not fold.