Gaining

The standard reply on here is to gain more visceral fat, they'll repost the "compendium of fat studies" that you can search on here, with interesting and proven diet tips that can help encourage more visceral or subq fat.

Ancedotally, losing weight and regaining it will make you softer, and gaining quickly and without losing much in between gains will make you more thick.

Additionally, and also ancedotally, belts and tight clothing can move your fat, so wearing belts below the belly throughout your day might help as well.

Unfortunately, there's not too much one can do to just force the ball bellied look if it's not natural for you. If I could stuff my next 50lbs straight into my gut I'd do that.

It's seems to me to be appropriate that you want to have a bell belly, that is firm and round. I have mainly visceral fat giving me the ball belly you want, but I would love to be a soft, jiggling ball of fat for a belly. Life isn't always fair to us, I guess.