The foot switches on the duo have a diameter of 8.8mm so most foot switch caps will fit. I actually have some on my own Duo. The hole in the cap is 10mm so it is oversized but they have 3 set screws so you can tighten it down and keep it central. They quite easy to find using the search term “foot switch cap”. The one’s I bought were the first option I found on Aliexpress I hope in the future we will offer our own as an accesory