If you're not hung up on the Hoya brand name, you can get neutral density filters in 2.4, 2.7 and 3.0 log densities (8, 9 and 10 stops) in the large square formats for Lee and Cokin filter holders, which will also make them usable on your other lenses. (The links are to product list pages at B&H.)
You could use a full-aperture solar filter that's appropriate for astronomy use. Orion sells many: http://www.telescope.com/catalog/search.cmd?form_state=searchForm&keyword=solar+filter
Advantage of this kind is that it would be safe to look through with your eyes.