I tried to see if I could go in and find something and tweak it but a: it was hard to find math at the level I wanted (either too low or too high) and b: modifying it meant diving into the code, and tho' I spent some hours trying to figure out a way in b/c ... I can learn code... that's clearly not something they've planned for. (I took our college class in Android App development instead... ) Now, I think they should put in (with two other organizations 'cause that's necessary) for doing that with the OER grant $$ that have recently been waved at us...
Programs like Geogebra (also open source) are *somewhat* easier to work with but ... still a real challenge to navigate. I love things like this one for making things visual https://www.geogebra.org/m/ua4yNy75#material/uUmKnBu6
I think I would do the "open pedagogy" part w/ maybe having students create a quiz from that "rounding" exercise, and discuss what kinds of problems it would be important to include on the quiz.
Your idea of having a quiz to add to the resource would probably meet "open pedagogy" standards. I also thought that in addition to the sim, which I guess most students won't be able to change, an instructor could include an additional open resource in the same plan that could be modified. In fact, an instructor could post the quiz as an OER, and students could both take and modify the quiz! Hmmm...
Congrats on learning coding! I'm jealous! Leecy