Many of the best HTC EVO 4G mods make use of a car mount holder that modders happened to have laying around somewhere at home. This is great for them since the project requires little to no extra cost, but everyone else has to go out looking for something suitable and then of course spend some cash buying it.
Fortunately, you can get an EVO car mount for as little as $2 and some loose change. The mount itself varies (windshield, goose neck, air vent, etc.) but since you're only after the holder, the style and quality of the mount doesn't really matter.
DealExtreme sells a decent-looking kit, complete with camera cutout on the holder, for $5.28 apiece and free shipping. If you buy 3 or more units, you can get the price reduced to as low as $4.29 each.
If you prefer the eBay route and don't mind dealing with sellers whose reputations may not be as solid as DealExtreme's, then you can get the same EVO holder with different mounting solutions for even less.
4ushop777, for example, sells the holder accompanied by various mounts for oddly specific prices like $2.34, $3.72, $3.94, and $4.64. The items ship from Hong Kong with free economy shipping, so you have to wait about a month for their arrival.