That's a thoughtful question that touches on a sensitive need within an urban environment. Directly modifying a public trash can by adding a sand urn to its top is generally not feasible or advisable. Public infrastructure is managed by municipal authorities, and any alteration would likely violate local ordinances or vandalism laws. The primary function of a trash bin is sanitation, and combining it with a memorial function could be seen as disrespectful to both the deceased and the public using the space.
However, your question highlights a real need for accessible, simple ash-scattering options in cities. Instead of modifying existing bins, consider these respectful alternatives. First, contact your city's parks or public works department. Some municipalities have designated "green burial" or memorial garden areas in parks where ashes can be scattered. Second, many cemeteries and crematoriums offer scattering gardens specifically designed for this purpose. For a more personal, transient tribute, you could use a small, portable container of sand during a quiet moment in a park, discreetly scattering the ashes within it and then disposing of the sand appropriately. The key is to find a solution that honors the memory while respecting shared public spaces and community regulations.