I personally think you're overthinking this - to gain (fat) weight - eat, eat often, and eat dirty.
In the USA, we have a saying called the 'freshman 15', where it is not uncommon for first year university students to gain 10-20 lbs during the academic year, since university food isn't always the healthiest (ie usually loaded with carbs and fats), and the students become abit more sedentary than previously (due to studying, stress, just being lazy, etc).
FWIW - you're still within an acceptable BMI (20 is still within normal limits). You state you've had eating disorders in the past (I'm assuming restrictive?) - so the biggest issue to gaining weight will be the psychological, and getting over the fear of the numbers and scales. That is difficult - I know. I was a former power tumbler and did competitive cheer in high-school and college; like alcoholism, it's very difficult to get over the psychological damage of previously having an eating disorder (15 years later, I still struggle). You just have to accept that the fat you put on will mean something in the long run (hopefully, it will be a controlled fat gain, and not morph into overweight or obesity, and when needed, you can safely diet down, if needed).
Pizza, grilled cheese sandwiches, late-night snacking, instant-noodles (Ramen, Cup O'Noodles, etc) - basically any cafeteria food that isn't a raw leaf salad - are calorically dense foods with little true nutrition; those can pack on the fatty weight. Just be careful it doesn't spin out of control. Maybe some sweet tea and/or non-diet sodas (seems to work for the states in the southern USA, where overweight and obesity are serious public health issues).
As an aside - for the past 18 months I've been doing alot of lower body/leg work in the gym (weighted squats, deadlifts, weighted kickbacks, etc) - gaining some leg and butt muscle has also helped in creating the allusion of a more feminine form, in addition to actually adding some weight. But if you only want fat - just eat badly and supersize everything, but be careful and methodical and don't let it get out of hand.