UPDATE (4/10/2019): The OpenSCAD file can now be edited and exported for laser cutting.
UPDATE (2/23/2019): Added OpenSCAD file to allow customization.
UPDATE (2/8/2019): Added BT-50 to BT-60 centering ring.

These are 3D printable or laser cuttable centering rings for model rocket engines. Attach it in the same way you would attach a cardboard/balsa centering ring.

If you need a size of ring that is not available here, please use the Customizer app or download and edit the OpenSCAD file. You can export the ring as an STL for 3D printing or an SVG for laser cutting. If you decide to laser cut a centering ring, cut from 1mm thick paperboard stock.

NOTE: I have found that using the OpenSCAD app is much more effective than the Customizer (since Customizer is often very slow and hard to manipulate).

Here are the outer dimension of body tubes - pulled from estesrockets.com - to aid in customizing the centering rings.
BT-5 - Inside dimension: Outside dimension: 13.8mm
BT-20 - Inside dimension: Outside dimension: 18.7mm
BT-50 - Inside dimension: 24.1 mm Outside dimension: 24.8 mm
BT-55 - Inside dimension: 32.6 mm Outside dimension: 34 mm
BT-60 - Inside dimension: 40.5 mm Outside dimension: 42 mm
BT-80 - Inside dimension: 65.7 mm Outside dimension: 66 mm

Most rings are 1mm thick. If you need the ring thicker, non-uniform scale the Z axis in your slicer OR adjust the height variable in OpenSCAD.