One of my bands left a padded bass drum cover on my bus, after months of trying to find out who left it, I suddenly realised it was similar in proportion to a Brompton.
Before our ride today, tried GF's Bromptom in it, & it fits a treat!
With room each side for clothes etc., it would make a very good travel bag.
It has one carrying strap.
They can be quite expensive new (depending on the make) but I've seen them on Fleabay for around £15.