I've seen very inexpensive mounts on Ebay that may work for you, especially since you'll use it while in park versus the time and materials of building yourself.
I chose the RAM-mount because it is sturdy. I like the pivots and locking points and I need my laptop to do different things, while I'm driving, and while I'm sitting still. It's also a nice touch that the engraved "RAM" logo matches my RAM truck. I'm a project manager who visits cell sites on a daily basis so my truck is my mobile office.
I've had a couple of Jotto desks over the years and the hardware they use at the locking points strips out and the whole mount moves afterwards. Had my laptop slamming into my dash and/or radio because of this. Also I don't care for the velcro straps or cables they use to secure the computer, it will damage the laptop - this is why I won't recommend them.