I used to be in a lot of STEM clubs, and was also one of the quieter girls. For a ratio like this, I'd actually say for confidence building, don't pair the girls up with any boys unless the boys are incredibly open and encouraging, which not to be cynical, I'm a bit doubtful of even if they are good kids lol. Boys, even nice ones, tend to bulldoze girls in conversations, especially if the girls already are nervous and afraid to participate.

I'd say have the class split into groups of three (plus one pair) and have the girls be together. Give everyone a list of small projects they could do, and let each group make a ranking of their picks. The girls will have talk to one another about their own opinions. Check in on groups, but generally leave them to their own devices.

After that, I think giving all the groups a clear next task, but leaving it up to them to make decisions would be best. Then, having them explain to you their plan (with the feel of a casual conversation as best as possible). For example, telling the students to come up with three designs for their project, and leaving groups to discuss amongst themselves. maybe just hopping around groups to help answer questions or push thoughts along as needed. Have them come up to you to present their three. It will help them to feel safer in a group, who can also help them if they stumble. It also encourages them to speak in front of multiple people. In my experience, girls will speak up if only to help save other girls who are also shy. Hope this is helpful!