The retainer is usually the last thing you glue on.
This way, the rear-most centering ring can be placed / positioned to help get things aligned. Then it is removed to apply internal fillets. Then finally glued in place. Then glue on the retainer.
But this is how we all learn!
Another helpful hint:
If you are using buttons, add a piece to the centering rings, another piece of 1/4" ply or... Sand to match the CRs! Align them when you glue the two CRs onto the MMT tube. Then mark off on the BT where these will be located in the BT. Once it is all together, you now have two 'thick' points to mount your buttons. Drill your holes, then use wood screws to mount them buttons! No need to futz with blind nuts or ..