Kaapland or Kaap Republiek is probably the best bet (though Zuid-Afrika is also a very strong possibility). But I also see it as incredibly unlikely that the British would hand back the Cape Colony - they need it to keep the routes to India clear.
They may give it up in exchange for a permanent lease at Simonstown. It's the naval stations they want after all, not the bother of administering boers and natives.
In terms of names, maybe something biblical or classical? NieuwKyrenaika? Nieuw Canaan?