I know the ladder is rated for a static weight but always wondered what the rating is for dynamic (while in motion). Your thought of supporting items off the bumper would help with those heavier items, but this distance and depth varies by build and I am not sure you find prefabricated attachments.
If you can find a tray to fit the items you want to carry, you should be able to fit diagonal metal supports above or bellow it with minimal fabrication tools (drill and saw). You can use insulated clamps to the ladder uprights rather than drilling them and possibly making them weaker.
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