Building a robot from scratch in an educational context can be a challenging prospect. While a multitude of projects exist that simplify the electronics and software aspects of a robot, the same cannot be said for construction systems for robotics. In this paper, we present our efforts to create a low-cost do-it-yourself construction system for small robots. We have created three different construction systems (laser-cut screw connectors, printed friction-fit connectors, and printed hybrid connectors) using small aluminium T-slot extrusions, based on prior work done by Industrial Design college students. Eighty-six secondary school students and 35 teachers tested these three systems during a five-day robotics contest where they had to build firefighting robots. Follow-up questionnaires and an expert evaluation were used to measure the usability, affective appraisal and functionality of the three systems in order to determine which system should serve as a basis for further design iterations. Overall, a clear preference was shown for the hybrid system, which relies on its interlocking shape as well as on a screw connection to create robot frames that are both quick to construct and very rigid once assembled. We believe our work represents a solid first step toward an inexpensive, "hackable" construction system for educational robotics.