Brompton Rain Resistant Cover for Shoulder Bag

I recommend this product

This cover is well made and bright.
I was buying for a non-Brompton messenger bag, for which bright waterproof covers are hard to find.
However, it seems as though the Brompton shoulder bag attaches at the base to something on a Brompton, because there is no way for the shoulder strap to come out. There is only the hole at the bottom. So, this cover should work well for my rack bag, but not the messenger bag. I am still looking for a bright fluorescent, reflective cover for my Timbuk2 messenger bag. Timbuk2 make black ones. Strange.