A few months ago, 12-year-old Peter Costigan came home after getting a haircut. The part in his hair, usually on the side, had shifted to the middle. His dad, Mike, a longtime Giants fan who grew up in New Jersey before moving to Wayne, Pa., thought, “He looks exactly like Ben McAdoo.”

A family outing, with four Costigan sons wearing Giants gear accompanied by mom Courtney and dad Mike, to the Giants-Eagles night game in Philadelphia on Dec. 22 — Peter’s birthday — ended in disappointment. The Giants lost a shot at the No. 1 seed, though they qualified later that weekend.

Making it worse, Peter had gotten near the players’ tunnel and Giants safety Landon Collins tossed him a towel — but the souvenir was snatched away by an adult. Well, there’s always next week.

But that meant a game in Washington. And front-row seats notwithstanding, the game had little meaning to the Giants, so Mike Costigan decided to forge his own energy. Mini-Mac was born.

“I said to my wife, ‘We’ve got to come up with something to make this a memorable trip,’ ” Mike said. “So we thought, ‘How about we make this our thing? Let’s dress Peter up as Ben McAdoo.’ ”

Peter loved the idea.

“I thought, ‘This is going to be fun because we’re going to have front-row seats at a Redskins-Giants game,’ ” Peter said Friday over the phone after school. “Last year when we went to the Redskins game, the Giants lost and they didn’t make the playoffs. So this was pretty cool.”

A pair of earphones used for watching movies in the family minivan became an NFL headset by using duct tape and a piece of wire. The family had a picture of McAdoo and copied it. Dark glasses? Check. Moustache? Check. Hair — don’t even need to think about it. Play card? Now some help was needed.

The family went to a Wayne, Pa., landmark, Minella’s Diner, and purchased a $100 gift certificate to get a menu — McAdoo’s monstrous playcard is often compared to a diner menu.

“Got the gift certificate to secure a menu because it sounds really stupid for what we were looking to do,” said Mike, a partner at Ernst & Young Financial Services. “We chopped it up in little pieces.”

With an added touch of Post-it notes.

All of this brought a reaction from Courtney: “You guys are insane.”

It brought a greater reaction at the ballpark. Guards snapped pictures of Mini-Mac. Television cameras noticed. Social media exploded.

McAdoo ran over and high-fived the boy, who said the most excitement was, “When Ben McAdoo gave me the high five,” but that the most special moment was “being there with my family.”

The youngster is in for a bigger treat, one of which he is not aware, as McAdoo has awarded him a game ball that the team has mailed to him. That should be a hit with the remainder of the Costigan brood which includes sons Michael, 13, Tommy 11 and Bobby, 4. All attended the game with Michael and Tommy appearing in most pictures.

Soon came the texts and calls.

“My friends were at the diner that I used the menu for and they saw me on TV and said, ‘Is that Peter Costigan?’ ” said Peter. “I didn’t have my phone but my brothers’ phones and my mom’s and dad’s phone were all blowing up with texts.”

It makes him something of a celebrity sixth-grader back home, where he plays three sports, including football. Plus, when the Super Bowl was played at MetLife Stadium in 2014, Peter won a salsa contest hosted by Victor Cruz.

“I wouldn’t call it famous, but it feels pretty good,” said Peter whose initial reaction to it all was “Oh, my God, there’s a picture of me on Giants social media. That’s crazy.”

When the game ended, Giants players gave the boy towels, gloves, hats. Collins signed the menu/play card.

Dad Mike said Mini-Mac will retire. It was something to lift spirits, which it did.

“It was a beautiful day,” Mike said. “With a joke, typically someone is the butt, on the wrong side. This was innocent, fun. Sunday, everybody won.”