I refill with watered down hand-soap (about 3 parts water to 1 part liquid soap). The pump bottle is what makes it foam, it doesn’t have to be a special surfactant, just anything that is low viscosity.
Be careful not to refill above the level of the internal pump - if you do it can get flooded and the extra pressure can break the pump mechanism.
They have a thin plastic mesh in the pump mechanism that does the foaming. If you refill it with a mixture that is to viscous, the mesh can break and the pump won’t foam anymore.
Also, putting too much water in will temporarily stop the pump from working. The device needs a reservoir of air inside as well. If the pump mechanism gets flooded it may need to be left to dry out before it is usable again. Trying to force the pump when its flooded can also break it.
I occasionally play bass guitar and keyboards and have found Behringer’s studio-monitoring range of headphones provide excellent quality reproduction for around AU$50 - better than many AU$200 consumer-focused brands. Sennheisers are also good but more expensive.
Thanks, that page also links to Wonderbag which is far more practical to DIY. The branded ones are rather expensive (although to be fair that price includes the charitable contribution).
In any case wrapping a stockpot in silicone (to protect the fabric) and then even more insulating material should be straightforward.
At first I thought the first word was sarcasm and didn’t realize the company was called “Wild”