I have used all sorts of things in the past but Have found when you are relying on someone else to cast the bird a towl can cause a few problems, if the person is a old hand at it then thats fine but if the towl slips forward against the lay of the feathers then they end up looking a mess...
With bare hands you get the feel the bird and apply the right aount of grip to secure the birds, if done properly you get no slip or damage...
In my experince for what its worth a Harris doesn't struggle that much compared to accipiters and Falcons, when you get a real wiggler this is when a towel can cause a bit of damage....
ATB
Jasper