I believe wild Alaskan salmon is supposed to be relatively 'safe', and tastes great in the steamer!
Also, how about avocados? They're quite calorie/fat dense and should be relatively easy to digest. Today I combined the meat from a couple of avocados with two spoons of coconut oil and gave it a whizz in the food processor until smooth. Half an hour in the freezer and I had a tasty, green 'ice cream' snack that easily increased my calorie intake by 600 calories or so - good, healthy fats, and caprylic acid in the coconut.