Although I do not wish to gain weight, I do have trouble maintaining a set weight that I would like to live (and compete) at.
And yet, I am on a 16/8 or even 18/6 fasting schedule about four days per week.
The answer, for me, is almond butter. Tremendously dense in calories and very easy to add 1/3 cup (or more) to a meal replacement shake. It's one of the easiest ways I have found to have a 1500 calorie meal that doesn't involve a tremendous bulk of food. Avocados are another food that fits this same bill.
 I am a "lightweight", or 168 pound, fighter as per IBJJF.