For a better slide into the upper stage tube, round off the top edge of the red coupler.
Before doing the CA coat below, test fit the coupler in the main tube. Sand the coupler sides if needed for a easy slip fit.
Slide the coupler into the booster tube and draw a pencil line around the top.
Coat the exposed area above the line with medium CA glue brushed on with a Q-tip.
Sand the dried CA with 400 grit sandpaper.
The treated side of the coupler makes a smoother fit into the upper body tube.
TIP: According to a recent TRF post, the coupler should be a very loose fit in the upper body for smooth, vertical staging.