I have some Crash Racers tracks and I was getting annoyed how often the cars would get up on the sidewall and either get hung up or go off track completely, so I designed a sidewall that can be added to not only stop that happening but also gives more stability to your track (as you can screww the sidewall design to your substrate) and an option to add additional fencing on top or even a billboard.
You can get the file for free at my Thingiverse listing here www.thingiverse.com/thing:5094031 - there have been a few people having issues with the download from thingiverse, so there is also a link to the file in a dropbox file in that listing as well.
You can see it here in Craig's (from Hot Wheels Boys) video after I sent him some to try.
You can download and print as many as you want, you just can't sell them or the file itself.
Let me know how you go with them :)