I agree with Yuna. There are many successful customized t-shirt businesses in the world. Thankfully they probably didn’t run a design sprint to evaulate their idea. This is my key issue with Design Sprints for evaluating new product ideas. Fair enough, you can be encouraged if all 5 participants explode with excitement, but just because 5 people are meh, doesn’t mean your idea is a bad one. You can find problems with 5 people and that has been proven with data. I’d like to see some data to prove you can assess product ideas with 5 people.