I don't think hacking something together would be very effective as the flexible metal blade of the scraper has to be forced flat against the floor to ensure it doesn't catch on any proturbances. It might be difficult to get a good connection between the handle and the scraper.
If hands and knees are not out of the question a heavy duty Skarsten scraper which is used in a pulling rather than a pushing motion might do it. Windmill Leisure and Marine used to carry them, or some of the other chandlers/marine suppliers, Western Marine in Bulloch Harbour etc. may have one
Don't like the sound of a steam cleaner. If the droppings are dry it is probably best not to wet them until the excess is cleared.
ISTR that dry bird droppings can metamorphose into unpleasant/nast/unhygienic dust so a good dust mask probably would not go astray.
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