Cheap snack cakes and pop tarts. So sugary, so unfilling, at least until you're well and truly full.

If you also like padding, put on multiple shirts. It restricts shoulder movement and puts more bulk on your gut.

Nutty Buddies to dip in Nutella or peanut butter.

Burger King- 2 Big Kings, 2 small fries, 2 cokes, an order of jalapeno poppers, and an Oreo shake.

As posted above, some really cheap calories (and overprocessed sugars) can be had at the "dollar" store. Those generic cookies, snack cakes, and pop-tarts can easily clock in at 1000+ calories a buck (or so). It's not "fast food" per se, but it doesn't need cooking.

All in all, maybe try eating a balanced diet, just a larger amount of everything.

It seems to very from person to person with tolerance, but I'll say that carbs take longer to break into sugars, then both become fat. Sugars should do so faster, so all else being equal, go for sugars.
That being said, neither option makes for healthy gaining.

Being fatter isn't something you can hide. That being said, if you don't want too much embarrassment, eat a bit less when family is around, and eat more when they aren't. Tight clothes likewise should be avoided with family. Get a size or two up if you can.

MinMaxing your money per calorie is important. Go for Burger King's 2 for 10: 2 Big Kings, 2 small fries, and 2 Fantas for $10 is the most calorie dense choice.

Also, as thighs rub and belly and breasts hang more and more, use a powder under those rolls. Any skin on skin will develop humidity.
Also tuck underwear under belly hang, if you don't already.