I would 2 part epoxy one side trying to work the epoxy as deep as possible. Let it cure fully. Then on the other side start with a superglue or something that will run down into the opening without having to be pushed. Might try 2 or 3 times waiting for a full drying time inbetween. Then cover it all with another layer of 2 part epoxy. Wait till it is fully cured. Sand it all smooth. Finish with touch up paint. The reason for so many steps is an air void is your biggest enemy at this point. Racks generally are not really abused and they really don't support a lot of weight. But an air void with trapped air will expand and contract as the air is heated and cooled. This could put pressure in an old crack that could cause it to crack again. If you lived closer I could get you some industrial Z1 resin, but it is hazmat for shipping purposes and waste unless it is fully mixed and cured, at that point you can eat off it! It also is extremely good at filling voids since we use it in large turbine driven steam turbine generators.