Check out KILDWICK.com for some awesome d.i.y. kit's or already assembled composting toilets. They come with everything you need for less than half the cost of the leading Natures Head toilet :
Kildwick Compost Toilet Upgrades for our BUS CONVERSION//12V Fan and Urine Siphon - S04E19 | And the bathroom build begins! First up - some upgrades to our composting toilet with Kildwick.com Link
What is a Composting Toilet and How Does it Work?
Kildwick® Composting toilets – your eco-friendly camping toilet
Best Composting Toilet For Van Life? - Kildwick MiniLoo Review - YouTube