At age 12, Bella Lin spent an hour every week "scraping, scrubbing and power washing" excrement off her two guinea pig cages.

"It was essentially a [mini] porta potty," Lin, now 17, tells CNBC Make It.

The experience was taxing and gross enough to prompt Lin to design her own cages: first on notebook paper, then with bricks and plastic in her backyard in Sunnyvale, California and now in a factory in Hangzhou, China.

Lin is the founder and CEO of GuineaLoft, a small pet accessory brand that sells on Amazon. GuineaLoft's cages are made with acrylic walls and a disposable wax-coated paper bottom that makes them easier to clean. She launched the cages in November 2022 after about a year of prototyping.

This year, GuineaLoft has brought in roughly $71,000 a month in revenue so far, according to documents reviewed by CNBC Make It. That's up more than double compared to last year, when the company brought in $34,000 in average monthly revenue and made nearly $8,600 in profit per month.

Lin credits the explosive growth to one event: Last October, she won $10,000 from a pitching competition at BizWorld, a project-based entrepreneurship program. She used the funds to buy an acrylic laser cutter, which dramatically sped up production times and kept the signature cages in stock.

GuineaLoft employs six full-time workers to develop, build and test the products, but the business remains Lin's side hustle. She recently graduated from high school and is moving to The University of Chicago where she plans to study economics in the fall.  

Here, Lin discusses how she built the side hustle and how she plans to maintain it as a college freshman.

Home-grown mentorship

GuineaLoft isn't Lin's first side hustle. Since age 7, she was "always trying out different ways to bring in little amounts of money" — from lemonade stands to selling hand-knit scarves — and was largely encouraged by her dad, a computer programmer who had worked with start-ups.

Once, on the way to water polo practice, Lin looked up a pair of Lululemon leggings and was shocked at the price. She recalls her dad asking "what do you think the markup is on this?"

The pair looked it up and found that most leggings made from similar fabric to a pair of Lululemon's cost about $20 to produce. The conversation prompted TLeggings, another one of Lin's side hustles, that brought in $300,000 at its peak in 2020.

"I think with a lot of parents, when kids are younger and they express an interest in start-ups with something trivial like yarn or slime, it's easy to shut that down," Lin says.

"My dad … always treated me as an adult, as somebody who was almost working alongside him."

Running GuineaLoft

It took about a year to take GuineaLoft's signature cage from prototype to Amazon, where Lin listed 100 of them. The first batch sold out within two weeks.

Sales have continued to climb, thanks in large part to the now three acrylic laser cutters used in manufacturing, increased marketing and an expanded line of pet products. GuineaLoft's Amazon storefront sells hay feeders and no-drip water bottles for guinea pigs, but also acrylic cages and accessories for hamsters, rabbits and birds.

The expansion hasn't come without challenges. While Lin considers herself to be a "veteran guinea pig owner," she's had to gain experience designing cages and products for the smaller pets.

The company went through "at least five iterations" of its hamster cage: "To be completely honest … it got bad reviews," Lin says. Customers reported their hamsters were escaping, so GuineaLoft adapted its design.

'I'm kind of just the guinea pig girl'

Outside of BizWorld, Lin's friend circle is void of other entrepreneurs: "To them, I'm kind of just the guinea pig girl," she says. "I like occupying a niche."

For Lin, finding that niche has been the secret to her success. GuineaLoft is her first profitable business because she was able to fill a gap in the market and innovate on her original ideas, she says.

Currently, Lin works 30 hours per week on the side hustle, and is enrolled in two college courses. She originally thought she'd take a gap year to focus on GuineaLoft, but says she's hoping to learn more and meet people who can potential help her grow the business.

Despite the company's profitability, Lin hasn't started paying herself a salary, she says. Her parents and a scholarship are covering her tuition at The University of Chicago.

Lin puts the money back into GuineaLoft, with plans to get products on brick-and-mortar shelves soon. "Opening in-person stores is a really big dream of mine," she says.

"The ultimate goal that I have for GuineaLoft is just to be the recognized small pet brand."

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