Serrated locking part

You may be able to use Nord-Lock washers for your project. The holding power is quite strong, as it is based on the torque of the bolt/nut, not on the washer.

As you can see in the image from the web site, the surfaces of the washer are serrated in a manner similar to the detents in your image. The two parts are interlocked in such a way that rotation creates a force opposite to the fastening torque, effectively tightening the connection.

I've used this type of washer in a high vibration environment and found them to retain the fastening torque indefinitely. A bit more expensive than ordinary washers, but for your application, perhaps cheaper than a ground-up fabrication.