Update: self stacking version, you don't need an alignment ring.
Customizable PCR tube rack.
1) Print both ends (almost like bookends).
2) Laser cut any hole pattern (I use a Glowforge) OR 3D print an insert (3mm thick) with your own hole size or pattern.
3) Use the 3D horizontal Alignment bars if you want several racks to line up in a row.
4) Use the Vertical alignment rings to stack the racks vertically.
5) Although these parts are press fit, you can glue them in place.