Error message for custom validator

Hi all.

I have created a form request validator which extends FormRequest. I have several 'standard' rules in use and a custom rule, which I am returning from my rules function as follows:

    public function rules(): array
    {
        return [
            'stake' => 'required|numeric|gt:0',
            'betting_event_ids' => [
                'required',
                'array',
                'min:1',
                'max:14',
                new NoDuplicateFixtures
            ],
...

I am able to set custom messages for the 'standard' rules, but not able to set a message for my custom rule:

    public function messages(): array
    {
        return [
            'betting_event_ids.required' => 'Invalid accumulator - No selections',
            'betting_event_ids.max' => 'Invalid accumulator - Accumulators are restricted to 14 bets',
            'betting_event_ids.NoDuplicateFixtures' => 'a test',
        ];

In the above, 'betting_event_ids.NoDuplicateFixtures' => 'a test',, has no effect. It's clearly not recognising my custom rule class name when setting the errors, which doesn't surprise me, but I can't see how I refer to it otherwise.

I know I can set the message in the custom rule class itself when calling fail(..., but I would like to keep all messages in a single place, and be able to use the same rules in different places with different message.

TIA

:wq