Oat meal is great for bulking.
Buy the massive bags of it and you can just eat it raw like any other cereal.
Pb and nuts are always safe bets when bulking. You've got the milk down which works big time.
Canned chicken is actually alright but it's quite low in fat.
If you can stomach it I recommend canned salmon, you will never find a better bulking food. Eat two cans and boom 600 kcal 80 grams protein, 0 grams carbs, 30 grams fat.
Canned beans are also a god send, things like chick peas being the best. If you can stomach it again, pour a little olive oil over a can of chick peas and just go crazy.
Also don't over look butter and cream.