There is a worldwide movement to brand everything under the sun William and Kate: Pez dispensers, air-sickness bags and, of course, sapphire engagement rings. Yet when the second in line to the British throne, Prince William, finally ties the knot with longtime love Kate Middleton on April 29, at least one royal fan will be celebrating more than their marriage and the retail bonanza it has sparked. Becky Schupp, winner of The Post’s contest to design a royal wedding dress, turns 25 that Friday.

Though she has long followed the British royal family, Schupp hadn’t noticed the coincidence when the couple announced their wedding date in November. It wasn’t until she was looking at the dress competition that “I saw the wedding date on your Web site and thought it was kind of funny.”

Aspiring designers submitted entries from Feb. 18 to March 11, and readers narrowed down the 56 sketches to five finalists. Schupp’s design garnered 38.2 percent of the vote.

Schupp, a resident of Helena, Mont., has wanted to be a fashion designer since she was 11. Her hobbies include sewing and drawing, and she studied apparel design at St. Catherine University in St. Paul, Minn. She moved to New York after graduation in May 2008 but returned to Montana after a couple of years and is trying to build her portfolio while looking for other work. She hopes to return to New York this fall.

Fashion is “a different sort of art form,” says Schupp. “It’s fun, exciting, a good way to express yourself.”

Washington Post's Royal Wedding contest winner Becky Schupp. (Courtesy Becky Schupp)

In designing the gown that won The Post’s contest, Schupp, who is single, did think a little about what she might want to wear at her own wedding one day. But she focused chiefly on the royal bride-to-be and thought about outfits that Middleton has been photographed in.

“I think her style is very young,” Schupp says, but “definitely classic and clean. I wouldn’t say she’s a risk-taker, but she definitely has her own sense of style, what’s appropriate, [what] makes her look good.”

So how to create a dress for a young, modern woman in a historic occasion and setting? Start with sleeves.

“I assumed that in Westminster Abbey you’d kind of want something a little more modest, like covered shoulders,” Schupp says. The church where William and Kate are to marry has been the venue for British coronations since 1066. A “royal peculiar,” meaning that it is the chapel of the British sovereign — and “exempt from any ecclesiastical jurisdiction other than that of the Sovereign,” according to its Web site — it is also the place where William’s grandmother, Queen Elizabeth II, and other relatives were married. Schupp went for long sleeves, but in a sheer fabric.

The rest of the ivory gown would be a heavier silk, she decided, with a very full skirt. (Full or loose skirts were features of the wedding gowns of the queen; her sister, Princess Margaret; her daughter, Princess Anne; and Sarah Ferguson, who took four minutes to walk up the aisle of Westminster Abbey for her 1986 wedding to Prince Andrew.) Schupp ruled out embellishments other than a band at the waist — pale pink could be a nice touch, she thought — and focused on “architectural sort of layers” with the skirt, along with a sweetheart neckline.

The total effect? “Kind of classic but still has a modern twist to it.”

Although Schupp read magazine articles about the couple for years while they were dating, she won’t be fancying the groom when William makes his way up the aisle this month. “You know, I was more into Harry, to be honest.”

The contest results:

1st Place: Becky Schupp, Helena, Mont.

2nd Place: Molly Beich, Anchorage

3rd Place: Annie Drury, Washington D.C.

4th Place: Anna Nieman,Wellesley, Mass.

5th Place: Jill Pylant, Atlanta

Honorable Mention: Jennifer Hart, Arlington