The 48-unit high-rise HDFC co-op at 1715 Nelson Avenue in The Bronx has a new amenity to offer its residents: Nine custom-built steel storage units from Bargold Storage Systems. Under Bargold's rental plan, the company absorbs the full cost of installation (estimated at $3,000 per unit) and then rents out the units to tenants itself, sharing a 25 percent commission with the co-op.