I searched and did not see this anywhere so I thought I would post it up for others.
I just made a personal lap timer. You modify the Microsizers timing system. With the Microsizer lap counter, laser pointer, and telephone wire you are set. The Microsizers lap counters are getting hard to find but you can get them.
Here are instructions.
For $25 and about an hour of time I have my own very portable system. It can do timed races up to ten minutes or lap races up to 500 laps. It only will tell you overall time and fastest lap when you are done. While you are running it displays the last laps time. So if you have a friend nearby they can write down all of your lap times. The only real problem is it uses a laser so it counts a lap every time the laser is broken. You can really only count laps with one car on the track at a time. Unless you create a way that only your car will break the laser, with something like an extra long antenna tube and having the beam up above everyone else.
I bought a decent quality laser pointer and used an aluminum tube for the detector. With this I have been able to set the laser and detector 15' apart and it detects a pen being waved through the laser at high speeds. This setup should have no problem picking up an antenna tube on a 8-10' wide track. If it would stop raining I could go put it to the test.
Here is my setup right now.
I am just happy because now no mater where I am driving I can get lap times and work on things.