Recently discovered that RV waste gate valves probably make the ideal gunge tank valve. I say probably because the project I decided to go do after discovering this got really complicated but i'm close to testing a solution to interface it with a bucket. A company called "Valterra" makes them and they come with kits to actuate them with a cable handle. i've posted some photos on my profile of a setup i've been working on to open and close one pneumatically. They are pretty cool!