This is what I normally use for motor mounts. It weighs about 23 grams and is made of HDPE and FR10 G4.
I designed a 3d printable motor mount and printed it out of black abs. It allows me to locate the motor about 4cm further out on the arm. The downside is it weighs about 29grams. I know that doesn’t sound like much but it adds up quickly with the need for 6 of them in a hexacopter. I haven’t decided whether I am going to use them or not.