Made from an old barrel grill someone left at an apartment complex trash dropoff, and some aluminum pipe from a weather-destroyed small greenhouse. Besides some basic tools like wrench for bike wheel nuts and drill + drill bit to cut a hole in the pipe to fit on the wheel bolt, all I needed was a little welder to fit an attachment point behind the seat stem.
Included are some images of options I considered before realizing my best path forward. Make shit up and use trash to do it!
The grill shelf portion of this is a good potential option in that direction, though it is a little heavy. Maybe you could just bungee a couple pieces of wood tightly onto the existing rack to widen it horizontally.
I was thinking in a similar way, wood plank secured to the existing one for the occasional larger load, then taking it off when not needed.