Simplified version of:
https://www.thingiverse.com/thing:1847711

This version is designed for quick and simple 3d print.
To use with:
6x75mm bolt.
Standard 8mm pen.

Note: make a 3mm thread for two screws.
I did not use 100 % fill. I tried with 75% and it works.

28Apr2020
Added a step version. However I did only a conversion with FreeCad program and not sure is it a good one or not.