no official adaptors mate, there was a post on here a couple of weeks back of some company in Holland or something doing some that did the job, have a search.
also on talk audio forums one fella used some rear speaker adaptors for a toyota i think it was and they fitted, have a search on there too.
MDF rings or custom door builds are the only other way. I was going to do it, but wanted it all to look standard in the end just settled for 13's. Saved myself a lot of hassle