You want to make your backdoor persistence? One of the easiest thing is to just add it to cron. However, everybody knows cron! Of course it will be the first place to check by your sysadmin, and your malicious backdoor will be gone as easily as you put it on the cron.
Try using at. Of course if someone knows it, it is also very easy to detect and remove, but I'll bet there are far fewer people that knows that the command even exists.