Modify what you have? I would think the Ortlieb mount part's thickness
needs larger EZ wheels ,
to not drag the mount, on the ground, when folded

I saw, in my Trek dealer a rack bag with a push button and a catch ,
that integrated with their rack. Worked well ..

My Brompton, I only have an M3L, I have a Carradice SQR support,
on the seat post,
with one of their big saddle bags..

It comes off easily.. , nothing interferes with the 'kickstand' rear fold under..



...