I have done a variety of things for mobile ops over the years. Currently, I'm using a handheld in my car as I haven't taking the time to mount a mobile yet. I've also used a handheld on my motorcycle.
My preferred mount is the RAM Mount RAM-HOL-BC1U - High Strength Composite Cradle for Devices with Belt Clips. Wow, that name is a mouth full. Let's just call it the Mobile Mount. Here is the official picture from the RAM Mount site.
This holder has a diamond mount on the back. I used one of these for a long time before I figured out I needed a place to hang a speaker mic. To do this I simply mounted a piece of metal between the holder and the ball mount. I rapped the metal with para-cord to give the speaker-mic some grip. This gives me a great place to put my speaker-mic. After taking these pictures I see that I should have painted the metal before mounting it.
You will also see that I put some felt pads and rubber on the holder. This isn't really needed in the car, on the motorcycle there is a lot more vibration and having a good fit really helps.
Next I'll show a few pictures of the other parts. Here you can see the radio on the holder, a short arm, a ball mount and a Pro Clip center console base.
The Pro Clip bases are great. They make dash, A-pillar, and center console bases for most modern vehicles. The platform they provide is even has marks for the standard AMPS (30mmx38mm) hole pattern used by the RAM balls.
Here is the same system unfolded so you can see the components better.
Finally, a couple of pictures of the Pro Clip base. This particular unit clips into the top of the console in a gap between a tray and the cover. A tool is included to make space for the clip. It then extends to the bottom of the console and clips around the edge. There is a small amount of double sided tape at each end. I wouldn't want to put a bunch of weight on this, but for a radio or phone it is more that sufficient and works well.
I have some velcro straps I run through the rectangular holes in the holder to hold coax and the speaker-mic cable. This provides two functions. One, it keeps the cables close by when I remove the radio. Two, it provides a strain relief for the speaker-mic cable.