I'm not sure I see the whole picture, but do you support searching by container information (i.e. SKU information), or only high level properties?

Easiest way would be to use an extension method to add container type and size to the products.  It can just be a unique select on varitions size and variations container.  The result would be IEumerable<string>

SearchClient.Instance.Conventions.ForInstancesOf<ProductContent>().IncludeField(_ => _.Sizes());

SearchClient.Instance.Conventions.ForInstancesOf<ProductContent>().IncludeField(_ => _.Containers());

public static class ProductContentExtensions
{
    private static readonly Lazy<IRelationRepository> RelationRepository = new Lazy<IRelationRepository>(() => ServiceLocator.Current.GetInstance<IRelationRepository>());
    private static readonly Lazy<IContentLoader> ContentLoader = new Lazy<IContentLoader>(() => ServiceLocator.Current.GetInstance<IContentLoader>());

    public static IEnumerable<string> Sizes(this EntryContentBase productContent)
    {
        return productContent.VariationContents()
            .Select(_ => _.Size);
    }

    public static IEnumerable<string> Containers(this EntryContentBase productContent)
    {
        return productContent.VariationContents()
            .Select(_ => _.Container);
    }    

    public static IEnumerable<VariationContent> VariationContents(this ProductContent productContent)
    {
    return ContentLoader.Value.GetItems(productContent.GetVariants(RelationRepository.Value), productContent.Language).OfType<VariationContent>();
    }

}

Thanks Mark.  I'm going to give this a shot this morning and see if this solves my issue.  Since we actually want the search results to be SKUs in our case, I suspect I will want to use the same approach but on variants instead.  Either way I can follow what you are suggesting.

Quan, we do support searching by SKU information.  Our site has an auto-complete search that targets products as well as a more comprehensive search page that returns SKUs and makes use of facets.  It was this second search that was giving me issues because the facets are tied to product properties that were not included in the index for SKU items.

Then I second Mark's suggestion

To close this one out, we followed Mark's suggestion and only altered it slightly to get product properties indexed within the SKU items.  

To do this we added a method to retrieve the first parent product item as we always have just a single parent associated with a SKU.

public static ProductContent ParentProduct(this VariationContent variationContent)
{
     var productContents = ContentLoader.Value
         .GetItems(variationContent.GetParentProducts(RelationRepository.Value), variationContent.Language)
         .OfType<ProductContent>().ToList();
            
     return productContents.Any() ? productContents.First() : null;
}

Then used this method in the individual property extensions like so:

public static string Brand(this VariationContent variationContent)
{
     return variationContent.ParentProduct().GetValue("Brand").ToString();
}

Works like a charm - Thanks!