Sold on the internet for 30€ or so, which i think is a ripoff. So i designed it myself so you can print it if you want such a thing. Feel free to modify it for your need

Designed for 25mm round stock, bolt on with 4 M4x20mm hex bolts and nuts. 

Warning! This is an untested design as i havent had time to test it and no really need for it.