I designed a removable Yepp-rack-compatible rear basket that consists of a milk crate, some plywood for stability and a 3D printed bracket threaded for M6 bolts which hold the whole thing together. It can be attached and removed in seconds and is extremely secure.
You can find a tutorial for building this project here:
https://www.instructables.com/Removable-Bike-Milk-Crate-for-Yepp-Rear-Rack-With-/
The author marked this model as their own original creation.