My 04 Avensis still has a tape deck in it, which is never used. And I needed somewhere to mount my phone without doing to many alterations or messing around with the dash. This solution worked a treat. All you need is a coat hanger, pliers, a tape cassette, some tape and about 10 mins.

Step 1: Step 1

Cut a nice long straight piece from the bottom of the wire coat hanger and begin to bend it into shape depending on the size of your phone.
I hate having to take my phone out of its leather holder everytime I want to dock it somewhere, so I just measured the bends with the phone still in the leather case and so now the whole lot can just be dropped in snug.

Just mark where you want to bend and place the pliers a few mill below that point and bend.

( the wire in the demo photos is not very straight, I ran out of the good stuff making my first one )

The very first bend is the bit that will be in the front of the screen, stopping it from falling forward.

Step 2: Step 2

The next bend is the depth (thickness) of the phone.

The next bend is the width of the phone.

Now you should have a rectangle that your phone slides into smoothly.

Step 3: Step 3

Now make a bend just a little bit in form the edge, and down the way. This is the length that will run down the front of the phone screen. So you don't want to make this bend to far over as it will invade to much of your screen, just make it enough to stop the phone from falling forward.

Step 4: Step 4

Now at the bottom you need to bend it back the way. This is the bit that the phone will sit on, stopping it from falling through.
How far down do you make this bend?
I'd say about a 1/3 the length of the phone will be enough, but you could go up to half the length of the phone or more if you like, but then you will be invading more of your screen.

Step 5: Step 5

Now make a bend which will divert the wire up the back of the phone. This bend will be the thickness of the phone. At this point you should be able to slide your phone into the holder part.

Step 6: Step 6

Now you just need to bend it back across the top of the first bit and then bend that into the L shape that will get taped to the cassette.

Excuse the bendy bit I'm left with. But you get the idea.

Step 7: Final

Now you can see my first 2 attempts. One of them is the one I have described here, which is quick, simple and works well. The other is a second and more elaborate effort with a bit more stability.
Just tape the "L" shape bit to a cassette and slide the cassette into the Tape Deck. Don't push it all the way in, you dont want it to accept the tape and start playing it, just slide it in enough to hold in place.

Step 8: CD Player

If you have no tape deck and want to adapt for a cd player, this should work.

Its hard to be specific for the CD Player as the set up on every car is different. The opening will be slighter taller in some than others, the opening will be set back deeper in some than others, and whats above and below the opening in terms of buttons etc will vary so much also, so their is a lot of room for variation.

But essentially you want to use the weight of the phone to your advantage. When the phone goes in to the holder, you want the weight of the phone to tilt the hanger slightly forward so that the whole unit is being held in by its own weight.

Here I have quickly adapted my first attempt for the cassette deck and it sits nicely in CD slot of my mates Focus.
I went up to the shop with it in like this and it never budged.

Step 9: CD Player Cont.

You could also reverse it and have the phone sitting above the slot rather than below it, but be careful not to position it in front of a vent, otherwise you'll quickly overheat the phone.

The photo below shows when I just flipped the holder upside down, I couldn't put the phone in without re-bending the entire holder, but using my fingers I put plenty of downward pressure on it and it was very stable.

The diagrams (sorry they're sideways) try to explain the theory so that you can adapt to your own setup. You don't want to go TOO FAR into the slot as there are all sort of things that could happen, you could activate the rollers that usually pull the CD in, or maybe even damage the lens.
But by just going in about 15mm or 20mm you should be safe.

If you have Sugru you could coat the Pressure Points and the rubberised coating would add even more grip.