Fabricate "Side-A" and "Side-B". Also make the "Snap" piece which you can friction fit or glue into the slot on the Side-B shell. Adjust the depth of this piece so that when Side-A is mated to Side-B it is held snug.

Press fit and glue 4 small pins into the alignment holes on the Side-B shell. I used 4d finish nails cut down to size.

The included STL files here are for fitting into a BT-20 tube. After fitting, tape may be used to affix the casing by wrapping the shoulder and the tube. To use as a nosecone, the fitting tube should have a bulkhead and eyelet that's tied to the shock cord. Also additional centering rings on the tube will allow it to fit snugly into a larger rocket body. Alternatively, print the Customizable Rocket Nose Cone Coupler: http://www.thingiverse.com/thing:2302595 and screw in an eyelet.

If you want to use the Customizer to make a different size, these are some useful Estes rocket tube diameters (Ref: http://www.estesrockets.com/):

Inner diameters for fitting:
BT-5: 13.2mm
BT-20: 18.0mm
BT-50: 24.1mm
BT-55: 32.6mm
BT-60: 40.5mm

Outer diameters for shoulder:
BT-5: 13.8mm
BT-20: 18.7mm
BT-50: 24.8mm
BT-55: 34.0mm
BT-60: 42.0mm

• 2017-04-09: Fully parameterized the egg shell profile. The size of the egg shell and the wall thickness can now be adjusted in Customizer.
• 2017-04-12: Add a coupler feature to the exterior of the shell. Allows the use of a rubber band to secure the casing closed after the payload has been loaded.
• 2017-05-01: Removed exterior coupling ridge and created alignment pin and internal snapping feature. Shell halves hold together much better and external rubber band no longer required.
• 2017-05-07: Updated part design to work best with the Customizable Rocket Nose Cone Coupler: http://www.thingiverse.com/thing:2302595