After prototyping a number of crude stringed instruments I had refined how to attach the strings and tune them. I noticed that when I placed the instruments on a wooden workbench and strummed them the sound was greatly amplified. I tried it on a number of wooden surfaces and found they all resonated.
I tried holding the instrument against the wooden arms of the play structure I’d built and the whole structure acted as an amplifier. The corrugated metal roof also helped contain the sound.
I ordered actual guitar strings and strung them between eye on the support arms of the structure. After some tuning the strings sound great. Nylon core with metal wrapping gives the good sound. and I imagine will weather better. Steel strings give great sound, but I worry about rust.
I intentionally placed the strings on different arms so that one child can’t hog the whole experience. It also encourages collaboration because you can’t get the full range of strings with a single person’s reach.