I put laptop desks in 2 of my vehicles...
In my van, i just used a plastic milk crate, turned it upside down, screwed a piece of plywood on the top (formerly the bottom of the crate) and stapled a piece of non-slip shelf liner on it. My laptop set at just the right height for me. Also, I used drywall screws at the rear of the box to mount a dc-ac converter. I also have room on the front to mount a scanner and 2 mtr radio.
For my S-10 pick-up, I bought some 1/2 inch pipe and fittings.
I removed the nut from the left front of the passenger seat and mounted a pipe flange, then replaced the nut. I came from there with a VERY short piece of pipe, then a 90 elbow, a 3 inch piece of pipe, another elbow, and a 2 inch peice of pipe. Here I put a union fitting, then a 3 inch piece of pipe, a "T" fitting, a 4 inch piece of pipe, a 90 elbow, a "nipple" and another flange. On the flange, I mounted a piece of plywood and covered it with the non-skid shelf liner.
Also, coming from the "T", I can put another piece of pipe with a pipe cap that I drilled out and put in a bolt to use as a camera mount.
With the union, I can remove most of the assembly and store it in the back until I need it and the only thing left mounted is below the bottom of the seat so it is not in the way for passengers.
It sounds complicated, but it only took about 30 minutes to put together once I figured out the measurements I liked the best. To put it back together and take apart only takes about a minute!!