Parts should fit together after removing burrs from printing. Ultimaker 5 series use 0,35 outer wall Line Width instead of 0,4 mm in CURA slicer.
Alternative bottom and cap here Cosmos Retro Rocket No.2:
https://www.thingiverse.com/thing:6421400
Get The Commander Fire Smoke plume in one piece here:
https://www.thingiverse.com/thing:4715125
and "gcreate" rocket smoke both at 50% scale.
https://www.thingiverse.com/thing:427789
