I wanted to put my HTC Hero on my new car but I didn't found a phone holder that fits with this phone due it's a little curved where the buttons are. I've already found a good car holder, but a little expensive (19€). It's a magnetic holder made by an Italian company:http://www.tetrax.com/en and taking the idea of a magnetic holder I decided to make it by myself.
The result is a magnetic car holder that fits with any phone with a battery cover (sorry iphoners) and in any position without making any hole and with a very powerful magnetic strength.

Materials:
- Old hard drive or flat neodymium magnet.
- Cutter blades.
- Double side tape, thick for place the magnet in the car, thin for place the blades in the phone.

Let's start:

Step 1: Get the Magnets.

I've disassembled and old 3.5" hard drive and there are two neodymium magnets to drive the reading arm. To detach it from the metal plate you can bend this support with two strong pliers, it will pop out due it's only glued. I've used the two magnets together due the magnet force its higher. It's not necessary to glue them, they are magnets!
Be very careful with this magnets, they are very strong and you could hurt a finger by placing it in a metal part, or you can delete any credit car near them.

Step 2: Place the Cutter Blades.

I recomend the little ones, but you can use any cutter blade or thin material with iron. To help putting the blades you can fix the magnet by some tape behind the cover, the blades will be attached in place in the better position almost alone due the magnetic field. I recommend to put the blades in the same position than the phone will be. If you will use the phone mostly in horizontal then put the blades like this, but if you will put the phone in vertical then put the blades in vertical. Adjusting its size is easy, due you can break the blade to the size you want.

Step 3: Double Tape on the Blades

I've put a thin double side tape, but you can put a little balls of blue-tack or even a soft glue. This will fix the blades to the battery when you close the battery cover.

Step 4: Testing the Magnet Holder.

Once the phone is closed with the blades inside to see the results you can put the magnets with a thick double tape in any surface and test the behavior. You will notice that the magnets are very strong so be very careful when place the phone. The magnets must be very clean, if the chromed surface of the magnet is gone then put some tape over to avoid scratch the phone.

Step 5: Put Magnet in the Car.

Choose the better position for the magnet in the car and fix it with a strong double side tape. You will have the most easy, effective and comfortable car phone holder ever!!