How do you design a ventilated improved pit-latrine?

Look at "The design of ventilated improved pit latrines" from Mara, D. Duncan ; UNDP ; Technical Advisory Group (TAG), by the World Bank. The document is available at http://www-wds.worldbank.org/external/default/main?pagePK=64193027&piPK=64187937&theSitePK=523679&menuPK=64187510&searchMenuPK=64187283&siteName=WDS&entityID=000178830_98101903445788 or http://www-wds.worldbank.org/external/default/WDSContentServer/WDSP/IB/2000/04/24/000178830_98101903445788/Rendered/PDF/multi0page.pdf

If this doesn't work for you, try a hand grenade.