What do you mean by custom assistants because you can make your !<word> assistants with your own prompt and the model of your choice.

Do you want !joost to hit and endpoint of your choosing?

No, I was looking in the wrong place in the settings: "search" > "advanced" > "custom bangs". I see now you can assign a bang directly when you make a custom assistant. Very handy!

From https://kagi.com/assistant you can also click on the down "Model Dropdown Chooser" and there is an entry to make your own assistants.