My classroomis currently dealing with Rube Goldberg. I haven't done multi class collaboration, but I feel like for that portability would be key. As such I think you would want each team's work to fit in a letter box and have a set trigger, like all must begin with being triggered by a ball rolling in the front right lower corner and end by rolling a ball out of the front left corner. That way all the group's can place their project in a row and it can work.
In addition to teaching, I work with an organization (Northern Central Valley Stem Center) that promotes STEM education. I plan on suggesting working to get small stem kits produced with tiny home stem activities that could be done.
Really, though, such things could be done as a home project for a class lab, flipping the classroom so a part of the assignment is replicating the findings of the class lab at home with a sibling.
A perfect example is the candium lab. All it involves is pouring m&m's from a cup and counting the number of candies face up vs face down. A sibling would love to take part. The teacher could have the student replicate the in class experiment, but send each kid home with a fun pack of skittles to repeat their findings and see if the graph is still curved. They could have their sibling write a small conclusion to the experiment as evidence.
You question about students without siblings is a great point:
Students who don't have siblings could teach the lesson to a young relative or an adult family member. In the case of an adult family member it wouldn't be as helpful for a younger child, but the student still gets the benefit of the higher order thinking involved in teaching a lesson.