I read that spacers would be needed to keep the speakers from http://www.i-club.com/forums/showthread.php?s=&threadid=26129 , so without delay, I started on my first WRX project.

	I didn't want to have my door taken apart for too long, so I tried to figure out the adapter size I needed and make them up beforehand.  There weren't any drawings available so I took the dimensions I found on the instructions and modeled up the door opening and bolt pattern. Door sheet metal is represented in gray. I also modeled up the speakers so that I could have that bolt pattern as well. I then designed a simple round spacer (white). I will be making these out of whatever plastic I have available in the right thickness.
	The thickness of the spacer is critical, I didn't want the back of the speaker hitting the window. The Infinity speakers are 2 1/6" deep as measured from the mounting flange back to the magnet. The overall height of the speaker is taller than that. I read that the back of the speaker should not stick out more than 1.5". So I made the spacer 1/2" thick. I also don't want to make the spacer too thick and risk an interference in the front of the speaker with the door panel. No-one really mentioned how much room you have in front of the speaker before you hit the panel.
	After pulling the door apart, I made some accurate measurements on the door and the stock speaker. I had read that the mounting hole radius on the door was 3.375", but I knew that these things made in millimeters. The bolt circle is 172mm. This gives you a bolt radius of 3.386". I also measured the depth of the speaker, and it was  about 1.400", so I decided to fatten up my spacer to 5/8". To figure out how thick of a spacer you need, simply take your speaker depth, subtract the stock spacer depth. In my case, 2.0625 - 1.400 = .6625 or about 5/8".
	I also decided to change the shape of the spacer from 7.5" round to match the shape of the stock speaker. After looking at the stock speaker mounted in the door, there wasn't a whole lot of room at the bottom of the door and if my spacer was bigger than the stock speaker, I risk hitting something. I measured the shape of the stock speaker as best I could to generate the geometry of the spacer. I offset the mounting holes from the speaker holes as not to create a weak point and add symmetry to the part.
	I found some 5/8" black delrin sheet and went ahead and cut out 2 spacers. Once I had the design, it took me 5 min to generate a program, 10 min to mount the material on the machine (3 axis CNC mill) and set up the tooling and only 6 min to cut the parts out. It took about a 1/2 hour to make. The 4 hole bolt pattern for the Infinity is on a  6.050" bolt circle. They weigh about 0.4 lbs each.
	Next I mounted the spacers to the speakers with the supplied foam between the parts. I then soldered 5" leads on the speaker and added a male 1/4" spade connector to the end of that. I didn't have the actual mating connector, but the male spades seem to work fine. Worked up 2 of these and they are ready for installation in the car.
	I had to do a run to the hardware store to get some screws to make up for the thick spacer. I grabbed some #10 x 1 1/4" long sheet metal screws that worked great. The stock screws are 5mm, but good luck finding metric sheet metal screws at 8:30pm.  They just tap into some soft plastic inserts anyway. I hooked up the speakers and then used some self fusing silicone tape to ensure the leads don't short out and also to keep them from pulling out. It also helps waterproof the connections, but they already have some grease on them to do that.  I made sure I tucked the connection under the door window, but was able to look down the top of the door into the door. I could see that the speaker was away from the window and that the connection was not going to get rubbed by the window as it went up and down. As I put the door panel back on, with a flashlight I could see that there was plenty of clearance between  the speaker and the door panel grille. Its hard to say how much just by looking into the grill, but it seemed like there was at least room for another 1/4" or more.

I was pretty surprised how easy it was to pull the door apart and make the swap. I guess it can be attributed to DFM (design for manufacturability) that the auto companies have been embracing for over a decade.