Modified safety razor. The original design is excellent, but we had to change it because we didn't have the proper bolt to fit it. We ended up cutting the bolt too short and putting too much pressure on it.
So we modified the handle to make it thicker (because we made the inside wider) and made the top piece thicker. If you use a proper sized and length bolt, the original design will work for you.
We've included two versions: one with an added 1mm of thickness. Another that is 1mm thicker and incorporates a 12mm diameter washer to help distribute the pressure over a wider area. The handle also accommodates bolts with wider heads, which was the original mod we did by necessity because the bolts we have on hand didn't fit.
Print the top pieces upside down if you're not using supports.