I found that the guard rails that normaly come with a track are just eye candy, but nothing that really protects the car from jumping off the table. I used sheets of cardboard, but that didn't help much and also limited the view to the track in some corners. Some day i saw photos on the Internet where people were using perspex and i decided to give it a try. I went to a local hobby shop, where i got pieces of 0.7mm thick perspex (30x50cm big). I cutted them in two pices a 50x15cm. Problem was: How could i make them stand and hold a slot car at full speed? The solution was so simple i couldnt believe i did not do that earlier.
As you can see in the photo above i just cut off the small triangular pieces from the rails, so i could put the rails on the track and the perspex between them, it just works perfect. Advise: 0.5mm perspex is possible enough, the 0.7 perspex is to stiff allready. It has so much tension it lifts my track on the inner side a bit.