Return/replace that one bottom cabinet for the size needed, best option even if there is a slight delay in the project. It will drive you crazy otherwise.

There are probably 3" spice pullouts too. Depends on the cabinets you're using.

SRD provided the best advice. Either change out the bottom cabinet for something larger or use the filler strip. Only can decide if the end is the right place for the filler strip or if there is a different location where it might make more sense.

You are not showing us the complete kitchen but I'm guessing there should have been some fine tweaking on the layout, but you're too far along now for that.

Most 3" spice pullouts must be mounted between two cabinets. You could use a "dishwasher return panel" on the end and mount the spice pullout between the end cabinet and the DRP.