Gap Inc., Macy’s Inc., Nordstrom Inc. and American Eagle Outfitters Inc. are all missing out on a major boost to revenue and customer satisfaction because they don’t currently offer clothing that customers can customize online, Forrester Research Inc. says in a new report.

Meanwhile, Nike Inc., Converse Inc., Reebok and Lands’ End have already incorporated mass customization into their overall strategies and are examples of how retailers can tap into consumers’ desires to look beyond off-the-shelf options for new colors, designs, styles and fits, Forrester says. That’s because 39% of U.S. online consumers show an interest in customization, the report says.

Offering shoppers the ability to customize apparel products can solve sizing issues and build a retailer’s customer base. However, there are challenges. Many shoppers are unfamiliar with buying custom apparel, for instance, and aren’t necessarily confident in their skills as designers.

That’s why it’s best to give shoppers design assistance, Forrester says. For instance, retailers can offer through an online configurator suggestions on fashionable choices or some kind of model product that will serve as a starting point to help prevent consumers from feeling overwhelmed when they see a blank canvas, such as a shirt to which they can add colors or special collars and pocket treatments. The research firm also suggests retailers build a simple configurator that focuses first on fit, then on style.

Blank Label, which was interviewed by Forrester for the report, offers customizable men’s shirts and is the fastest growing merchant in the newly released Internet Retailer Second 500 Guide.

Gap is No. 24 in Internet Retailer’s Top 500 Guide, Macy’s is No. 17, Nordstrom is No. 34; American Eagle is No. 57 and Nike is No. 78. Lands’ End is a unit of Sears Holdings Corp., No. 7.