Instructions for rears
Okay so now that it is heading into winter and there are mushy leaves on the ground and soon to be sand on the road I decided to do this. The fronts I did almost like the OP, the only thing I changed is I raised the drill holes a half inch (to drop the flap height to 3.5"....personal preference) and added about an inch of material on the "outside" of the mudflap so they stick out of the car a little bit. I did this so that they would look a bit more like the Rally Armor flaps and protect the sides better. I think there may be a subtle difference here, as my car (and Rally Armor's) is a Speed 3 and the OP's is a regular 3, which has a different tire width. My Xice Xi2 tires I just got have actually have a wider shoulder and thus tread width than the stock RE050's too, even though they are the same size. Point is, it really felt necessary to have at least that much extra material to cover the wheel spray in a straight line. I added a little 60 degree cut on the top corner of the material that is sticking out just like rally armor, makes it look good.
Now for the rears. For god's sake take off the rear wheels! I just jacked it up a bit and it was a huge pain. Taking the effort to take the wheel off will make everything else easier. That being said it is possible to do it with the wheel on and the rear jacked up, just harder.
So first, the shape. The mudflap in the rear is the same total height, but due to the shape of the rear bumper add 2" to the "bottom height" of the flap before you start the curve to the top. There will be two holes to drill, I just positioned it carefully and guesstimatingly marked the holes by hand, redrilling a bit as necessary.
The holes are two rivets. With the rear jacked, use the 3/16" drill bit from earlier and carefully screw into the rivets. If you've never done this before the technique is moderate to high drill speed, LIGHT pressure. This will allow the drill to slowly seat and bite into the metal. Too much pressure or too low of a speed and the drill can "walk" off the rivet and onto your fender well paint. This is more true if you have the drill at an angle because the wheel is on. After drilling a bit the rivets will pop apart.
With the rivets off, reach behind the painted part and pull out the black plastic flap behind it so that it is out in front. Pull off any rivet backings, and then drill out the small plastic holes with a 1/4" drill bit. Replace the plastic behind the trim.
Use two 12-14, 1 1/2" plastic anchors for each flap. These are the next size smaller than the OP's front screw setup and come in a blue bin at Home Depot. As with the fronts, check for level, and I went with 3.5" off the ground. I also had about the same amount of extra material off the sides. One thing to note here is that the rear bumper is curving a bit inwards at this point, so my flaps aren't quite pointed perpendicular to the direction of travel. Rally Armor solved this buy using a bracket on the lower hole which re-aims the lower portion of the flap. I didn't make one but kinda want to, if anyone does let us know your design.
Some things to consider: I used a dremel cutting bit to smooth out the rough edges after cutting the trash cans with shears. Oh and of course it too 2 trash cans. 28 quarts is barely big enough, so I used all templates "upside down" with the bottom of the trash can being the top of the mudflap. Finding a large trash can might be helpful and economical. As for price, Things are a little more expensive here, so between 2 trash cans and the extra screws I ended up spending like $16 on all 4. Compare that to Rally Armor's cheap flaps for $40 (pre-shipping) that come with the brackets and some extra screws. Rally Armor is by no means a rip off considering how much more labor is involved in sculpting the flaps, but I actually kinda enjoyed that and $16 < $40. If I can ever afford the proper Urethane RA flaps I will probably get those eventually, but my car is protected and doesn't look kludged at all for now.
Finally, if there is enough interest in a template for the rear mudflap I can probably take mine off and make one.