Building your own composting head is not an easy task. You'll need fiberglassing skills, metalworking, and a good bit of ingenuity.
Before you start, it would be best to get a very good look at either an AirHead or Nature's Head to see how they work. We were lucky to see one at the Portland Maine boat show.
On the Mac 26S (and perhaps others) there is a fiberglass shelf that the port-potty sits on. For a composting head this has got to go. Get out the sawzall and cut out the whole shelf, and fiberglass over the hole to make a new shelf right on top of the ballast tank, about an inch below the floor. (first photo) This'll gain you about 5" of height.
The solid waste goes into a custom bucket. The space available is confined by the shape of the hull, so this is an odd shaped bucket. I made a styrofoam block to fit in the space, then rounded the corners over generously, wrapped that in polyethylene sheet, then covered it in several layers of fiberglass cloth and epoxy. Once the fiberglass is thick enough, tear out the foam and plastic sheet.
Then there's the auger. The peat and solid waste must be broken up and mixed. I found up a piece of 3/8” solid stainless steel rod, and using a torch, was able to bend it into it's convoluted shape for mixing the peat. A wooden handle and crank arm screws onto the end.
The solid waste bucket gets half filled with peat (garden supply) and a detergent bottle becomes the liquid waste receptacle. We bought a Separette toilet bowl and lid, which sends the solid and liquid wastes into their separate containers. A box made of vinyl board surrounds the sides, and a plywood top finishes it off.
Here's the final product, showing the hand crank for the auger. You'll also notice a small fan and PVC pipe out through the roof. In hindsight, I wouldn't bother with this extra ventilation, as there is no smell from this system.
Good luck with your build. For us this is the best way to go.
