Just a simple countersunk washer, works well for M5 and M6 bolts (M5 in the picures).

Super easy and fast to print. Use different materials and infields to get more strength or flex.

Easy solution for when you don't have alloy ones and a high tightening torque is not required.

Also great to ensure no one will overtorque something as it will break first providing the right settings were chosen for given strenght.