Why not buy a camping portaloo (sp) You just need to put chemicals into it and empty when necessary. They dont fill up too quickly if you dont fill them with paper and you can take them home every so often and tip down the main drain.
This is on the possibles list but I don't like the smell of the chemicals. The main thing putting me off is that the nearest I can park to my house is about 30 yards away & I would have to carry it up there to empty it.
Is there no main drain on the yard.... I do know that someone I know (deffo not me) puts hers on the muck heap (eewww) or deep into the woods (just put a flag in it)!!!