I have done it with a pop sock in the past, if you have such a thing. I think it's better to use oatmeal rather than porridge oats. I would try to tie it onto the tap whilst your are running the bath and then maybe also try to squeeze some water from the bag over him.
My DS is 20 wo and has pretty bad eczema. I was reading that an oatmeal bath may soothe him, but I have no idea how to go about it. Do I just soak the oats in some water and then put it in a muslin cloth and use that as a sponge for his body?? Do I need any special oats??
Sorry if this is too obvious and I'm just being stupid !