Another option is to just tie one on yourself. You don't need to buy one of those big old plastic things and clamp it to your bow. Just act like your going to tie in a string loop, but only do one half. Pull it tight then cut the string off and melt the end flush with the knot. Now you have a location marker that is really light and you can screw it up and down the serving to adjust it.

Some people use this same technique but they locate it so it touches the tip of their nose instead of the corner of their mouth. Since your nose is always in the same spot. It's too easy to move the corner of your mouth.

I have also done this in place of a peep. I put the knot where the peep should be and just aimed to the side of it. This actually works really well.

Paul