That looks like an effective conversion, though I admit that I wouldn't know a homo ergaster from an australopithecene.
But I wonder if you are making work for yourself by using Matabele, rather than some figures who are already naked - there are plenty of Greeks, Celts, Vikings (and I seem to recall a Foundry "Darkest Africa" pack called "bollock-naked tribesmen", if it's still available) which would cut out a stage of conversion - if I can use the word "cut" in relation to genitals without making everyone cross their legs, that is.