Hi. I have made a sliding shelf to sit under my desk to hold a musical keyboard. However, I'm having a problem with the shelf sliding. I'm looking for some sort of catch that I can put either on the top side or bottom side of the shelf (top would be preferred due to accessibility). I've searched numerous internet sites looking for something but haven't been able to find whatever it is that I'm after. I seem to remember something from years ago that was springed and allowed a flick of the thumb to lock it into place but, as I've no idea what it would be called it's hard to find. Any ideas please