Eppendorf test tube rack

I use these with a introduction practical biology, working with a micropipette

This is a small Eppendorf test tube rack for 6, ∅10 mm tubes
Or invert the rack and you can place 6, ∅8 mm tubes

Made from acrylic and four M3 bolts. And four M3 20 mm Hexagonal female brass Standoff/Spacer.

I'am a lasercutter man ; but if you're more an 3D printer, I think this design is better choice
https://www.thingiverse.com/thing:3377154