If you have a drill press, it is pretty easy.
For example, use a 1/2" rod, preferably metal, as opposed to a wood dowel that might be warped. Build up the diameter evenly with masking tape so that some BT-5 or 13mm tubing will fit over it axially for a snug fit, but leave one end uncovered for the drill press jaws to chuck onto.
Use the proper set of centering rings to mount the rings to the BT-5/13mm tube, to hold a tube of whatever diameter the egglofter body tube is. Say, a BT-50 (24mm body tube). Make it short so you will have plenty of clearance for the drill press. Slide the eggloft capsule shoulder/base into the tube, leaving about 1/8" of it sticking out at the bottom.
Slide the lower half of the egg capsule into place, bottom-up facing towards the coupler tube you want to glue on. Adjust the drill press so you can use the lever arm to lower the tube/coupler assembly (24mm in this example) to just the right spot on the capsule, and make use of this test-run to align the capsule dead center (left-right and fore-aft) on the drill press so it ends up being axially aligned. Let the arm up briefly to raise the coupler.
Apply a little bit of thick CA to help to tack-glue the coupler, then carefully turn the Drill press arm to move the coupler into place, touching the capsule and the CA, but not applying so much pressure that the coupler accidentally slides up inside of the tube attached to the drill press. Quickly spray some CA accelerator. Very carefully, rotate the Drill press arm which will not only move the mounting tube and couple up, but also will move the tack-glued capsule up. Very carefully slide the capsule and tack-glued coupler from the mounting tube (24mm tube in this example).
With the capsule base and tack-glued coupler removed, apply more thick CA, inside of the coupler, to glue the coupler to the capsule permanently.
Once you have the proper pieces of hardware to start the process, the above steps may go faster real-time than it actually took to read them (for sure faster than it took to write them).
NAR 18723 SR