At the end of construction, your structure needs to stand with no outside support of any kind. I will give you five minute time checks during planning and use a timer to set other time limits. Divide the class into groups of 5 to 6 students. Random assignment is one option for achieving heterogeneous groups. Careful construction of groups to increase their heterogeneity is another.
Heterogeneous groups are effective when working on ambiguous problems with no single solution.
Provide each group with an identical set of construction materials for their freestanding structure and the instructions. Your role is to ensure that no one touches the material during planning or begins building until the construction period. You may facilitate discussion, if called upon to do so, but you may not provide comments or suggestions. Following the construction period, applaud everyone efforts. If time permits, have each team discuss the following questions first, then share with the class as a whole.