There are after market caps available...some with hex heads. Bet a search would turn up a source, and some pics. I'll go check in a few...
Edit: Well, my search turned up zilch. I know I saw hex head plugs somewhere, but cannot find them now. Searched for diff plugs, diff caps, rear end plugs, final drive plugs, final drive caps...zero. Did see references to a cap sold by Murph (Murph's Kits), but could not find it on his web site.
Maybe somebody smarter than me can help.