I've yet to do this, as it'll require me purchasing a second skulltaker model and another jugger, and I have too many plaguebearers to paint before I can start on painting models for kicks rather than my imminent tourney list. HOWEVER.....
My plan for him is as follows. Rather than try to chop up and mangle what is such a gorgeous (and metal

) model, I'd run with him as is, and model him off the Juggernaught. Now I know that sounds like a lazy man's shortcut, but bear with me. It even says in his fluff that he would dismount when facing off with his chosen opponents.
-So what I do is firstly
GS a new neck for him, turned so he's looking up at the skull as he burns the flesh away, looking like he's just claimed a trophy instead of about to wing the skull at somebody.
-Then take a dreadnought base (as a model-less unit, it HAS no legal base size, so this is a reasonable choice), and some suitable specialish character. I'd go for the black templar "Emperor's Champion" just to bug my brother the
BT player, but you could easily go with a chaplain, autarch, warboss, and so on. Now, clip that poor bugger's head off, if it comes with a helmet, save that and put it next to the body, lying on his back on the base in a suitably beat up pose/condition. Clip the pile of skulls from the skulltaker's foot, and put him standing stoically with a foot on the chest of his slain foe, gazing up at the skull he has taken in a rediculously epic pose.
-To tie in his Juggernaught pimp-mobile, (and take advantage of it's already practically perfect pose), you make yourself a growing pool of blood from the neck of the dead character, from which your Jugger can be made to drink. The only change the jugger actually needs would be to put all of his hooves flat on the ground, as he looks like he's going full speed as is.
-Paint and Base as you see fit, and now your Skulltaker is ready to behead some 'fexes, C'tan and chapter masters! That's my plan, if/when I ever get around to it, I'll take some pics. I've got a conversion for Herald of Khorne on a chariot, as well as scratch-built blue scribes to post (and I didn't even cop out with the fantasy disc of tzeentch! perhaps to my detriment....). Maybe I should just do a whole thread when all 3 are ready to go....