We use LapMonitor for our parking lot racing. Works great for us.
Note that it does not work like MyLaps as it does not set up mains etc... we base our qualifying on number of laps and use hot lap as a tie breaker. (Kiss method of racing)
No issues with counting laps, even with 2 cars crossing the loop at the same time.

LapMonitor just came out with an upgrade allowing others at the track to view results on their cell phones.
And they have the ability to change the number on the transponder by 10 digits...