I used a Radio Shack 273-060a beeper however I don't see it in the online listing. The size seemed to be more important than the specs. When I was trying them out, they had varying dB ratings at various distances. What's online now only lists dB but not distance - check the package to see if they're marked. You'd want the loudest noise (highest dB) at the longest distance.

I used Velcro dots to hold it in place:

 

thanks mate

 

I have Modifry's Redline Shift Warning Module and swear by it. The unit and beeper are velcroed under the dash but it is still very audible with a helmet on.