Worked it out using this post:

https://our.umbraco.com/forum/umbraco-8/95915-umbraco-8-examine-indexes

public class SearchIndexCreator : LuceneIndexCreator, IUmbracoIndexesCreator
{
    private readonly IProfilingLogger _ProfilingLogger;
    private readonly ILocalizationService _LanguageService;

    public SearchIndexCreator(IProfilingLogger profilingLogger, ILocalizationService languageService)
    {
        _ProfilingLogger = profilingLogger ?? throw new System.ArgumentNullException(nameof(profilingLogger));
        _LanguageService = languageService ?? throw new System.ArgumentNullException(nameof(languageService));
    }

    public override IEnumerable<IIndex> Create()
    {
        return new[] { CreateCustomIndex() };
    }

    private IIndex CreateCustomIndex()
    {
        var contentTypesToExclude = new[]
        {
            ...
        };

        var index = new UmbracoContentIndex(
            "SearchIndex",
            CreateFileSystemLuceneDirectory("Search"),
            new UmbracoFieldDefinitionCollection(),
            new StandardAnalyzer(Lucene.Net.Util.Version.LUCENE_30),
            _ProfilingLogger,
            _LanguageService,
            new ContentValueSetValidator(true, excludeItemTypes: contentTypesToExclude));
        return index;
    }
}