I've got a 2018 ISG 3 and had the same issue. However, I actually bought a full size alloy wheel, removed the sub-woofer and placed it inside the spare wheel once it was in the wheel well. It fits perfectly and once packed carefully with some flex-foam, is snug as a bug. Have driven 20,000+ miles including a trip to Belgium over the last 12 months (the reason for wanting a spare wheel) and I've not had any problems. Have a spare wheel and a functioning sub-woofer for the excellent Harmon Kardon system. Win win.