You can make one or buy some type of strip or rings. The problems you have with them are of a couple different varieties. First off, they are made to be used like stadia lines in an old military type anti tank site. You' re suppose to beable to put the top line on the back and the bottom on the belly and if it fits cleanly it' s X yards(with the scale able to fit an 18 or 16 inch object in it. Well, all deer are not created equal in the girth department. Secondly, the scale has to be held the same distance from you eye every time to fit the animal properly. Equate it to looking through the eye of a needle or peep with it held against your eye and then again held at arms length... definitely a difference on what will fit between the lines. You could make one with tape, marking off some distances and holding your bow at draw. Stick the tape on the back of your bow near your line of sight. Have someone use a pencil and mark your tape. Deer are different sizes in different places. Does of course are much smaller in girth than a buck in the same local. Things to consider when making your scale. Around here I' d use a McKensie medium. In NY Adirondacks maybe between a medium and large. Iowa, maybe a JUMBO.[:-] You could just decide on what size in inches, draw a circle and then make lines for 20, 30 and 40 or whatever. It would be better than nothing. IF you hunt from real high treestands, the object you see as it gets closer is a little narrower and would throw your scale off a little. Pick a happy medium and you' d have something.