Tinkercad looks great - thanks for both the question and the answer.

I've also been looking for a program/ app that DS could use to create 3D graphs and shapes (he's addicted to a graphing calculator - www.desmos.com - and wants to up the ante). Preliminary google turned up a lot of things that need to be downloaded, and mostly paid for. Happy to do either for a good program, but leery of a bunch of stuff I don't know. Any recommendations?