I found this on a quick search of Google. I've seen a 5' LCD screen before for under 2 bills but dang if I can remember where. But check this out for now.
Now bear in mind having the lid down on a laptop will keep a lot of the heat inside the machine. Direct sun on the computer will not be good for it also. One last thing, power for these devices can overload a regular cigarette lighter socket. You may want to look into a power outlet from an RV shop to go up under the dash.