Keep it simple. Here is a link:
http://www.operationalmedicine.org/T.../FMST_1603.htm
There are plenty of other examples on the web. These systems have been in use by millions throughout history. Some easy to find items and a little fuel to make it burn.
The urine tubes are way simple after you have dug out the gravel pit. It doesn't sound like you are going to have many people using it over long periods, so the sand and gravel filtration probably will last a good long time.
The toughest part is finding a location that is close enough, but not too close.
The view in the photograph is very nice.
