Combine upper and lower parts using long M5x20 or M5x25 bolts.
Both upper parst connect using some 3mm filament or an M3x10 bolt.
The tubes are slightly tapered.
So you slide them on as until they sit tight.
Then secure the assembly on your spindle using an M5x25 nut and bolt.
Add glue to make things super secure and sealed if you want to.