Summary

Print using any material , I use PETG for the strength
the hole Diameter is 9mm and the outer is 14mm
you can either use a 8mm bolt as it is
or use a stainless steel thead insert for a 6mm, bolt, This is what i use on some clamps to use as a jig stop

There is a grove around the outer bolt hole
This so you can use a thin STAINLESS STEEL TUBE 16MM DI (1MM WALL) and has a 14mm ID wall tube to be inserted to strengthen and prelonge the life of the clamp from the tightening of the bolt