Trunk Mount and Trunk Bag for RR +

[jackkpts]

I removed the passenger seat from my new to me RR + as I will never use it. I want to add a good sized trunk bag for running errands, etc. I tried a bag that has velcro straps but the rear rack is much too wide for this type. I know there are bags that mount on their proprietary racks, but I don't need the whole rack, just a mounting plate. Any one come across a solution for this?  Thx