I was just about to ask the same question. I bought 6.5" speakers for my doors and they dont fit... im considering buying 6.5" wooden speaker rings & glewing them around the hole.
I put my speakers into the hole and it needs a half inch spacer and the ring is 1 inch thick so this will work... they're about £5
ill have a go and post some pics & info of what i did once the rings have arrived. It might not be great if you care about how tidy it looks behind ur door cards but tbh, your not gunna see an of it unless you take the cards off...