Id advise using wood anyway--those plastic brackets can break on install or driving. get the cutout dimater, and the trace the outline of the stock 6x9 and cut it out using a jigsaw, and then mount it to the door, or if your speakers dont have a large face mount them to black stock speaker mount. if you mount to the DOOR trace the back of the stock black speaker mount so you get the right holes for the bolts and just go to home depot and get some metal screws/nuts to match the stock ones (cant remember the size)
I can make some crude ones for you and mail them to you at your cost if you want--but they are very easy to make yourself
but if you WANT PROFFESIONAL my man is a cabinet maker who makes speaker boxes, baffles etc on the side--but these boxes are far from on the side work. he made a set of baffels for me the first time I installed my system and they were perfect just buy talkin to him. I still use those as models when I add different speakers--I just use a jigsaw, but his are still way nicer
anyway here is his website--he is a great guy totalk to and do biz with and from the pics you can see how nice the boxes, etc that he makes are
http://community-2.webtv.net/MarvsPlace/MarvsPlace/