I have not seen this particular part ready-made in O scale although from memory some Lionel aluminum passenger cars have had them as part of the body assembly. May be only limited help but Detail Associates makes HO 24” roof access ladder rests under part 6603, available from a few online suppliers. At that scale these parts would only be about 1/4” wide BUT that may be enough for your purposes depending on whether your car body is true O scale or somewhat smaller and whether they look proportionate to other roof details:
I suppose another option is to get ordinary O scale grab irons from a supplier like Des Plaines Hobbies and bend them to shape, which might save some labor compared to making them from scratch.
P.S. If you really want to go in for making a lot of these from scratch there are wire bending tools (basically pliers with shaped jaws) that might work. This one is from MIcro-Mark:
