I have a few pigeons who nest on the floor even though there are nesting boxes available for them. When they select a location on the floor it has always been against one wall so what I do is place three ordinary bricks in the area they have selected. This ends up as a square and they have always accepted that set-up and place nesting materials inside the area of the square. Another thing I do after they accept the location is to place a piece of plywood at an angle from the floor to the wall placing another brick in front to keep it upright.
This results in an enclosure and provides the pair with a sort of tent that they call home. It has always worked for me with Racing Homing Pigeons. I once raised King Pigeons and found that floor nesting is not acceptable as that breed in my experience is aggressive and will go after the young birds who wander from their nest. I stopped raising the King Pigeons after they pecked some squabs so badly that they were scalped.
As an added note, I use long pine needles for nesting material for my pigeons.