Shouldn't really need one to be honest the lid should radiate heat back to create even cooking, however there is a video on YouTube of one being used, quite simple just has a very thin drive section which goes between the gasket, there are then supports on the inner fire ring to hold it together.
Rotisseries arent just about all round heat, theres the effect of gravity rotating throughout the cook too, causing juices to flow all over the meat. Theres no up.