Remember that the porridge doesn’t need to be yummy since you’re not eating it! When I’m in the mood for this kinda thing, I buy ‘quick oats’ (just rolled oats processed into smaller pieces) and microwave them with water. You can modify the thickness with how much water you add. Of course let it cool down a little afterwards so you don’t burn anything sensitive haha.
i like it because it’s ultra squishy and you can make a huge quantity for really cheap.