I am planning to add a blackwater holding tank, not provided originally. Hunter sent me specs for a rectangular tank, for location behind the bath cabinet front, but I think a custom tank would hold considerably more and fit better. Wondering if anyone has done this? Also, I plan to replace the original fabric covered cushions with thicker, better quality resilient foam cushions covered with Ultrasuede, which should be much more comfortable and vastly improve appearance. I also plan to double the effective crankcase capacity (originally only about 2 liters) and greatly improve filtering by adding a Coast Guard approved Amsoil remote bypass filter. Thanks for any information/suggestions about adding a blackwater holding tank!Dave Ladely, Seattle, WA USAemail: DaveLadely@aol.com