Borbin the 🐱

The most critical part in the wheel is the spacer. It is build from stamped out sheet metal and machined to be glued on to the pipe (made out of 1"x0.12"x0.76" DOM Mild Steel Tube A513 the same as for the distance bushing).
If the spacer is not machined exactly 90° at the end, the bearings and the hub will wear out quickly and can make squeaking noises.

The parts.
Bushing - Bearing - Spacer - Bearing - Bushing - Cap Nut

It is important that the spacer has the correct size. If it is too small, the bearings are not fixed to each side and can slide, which can cause wear. If it is too large, the cap nut might have only one thread to hold which could break.
To check for a good fit, use a round plastic with the same dimension as the bearing. The plastic replacement for the bearing can easily slide out. It is not pressed in as the metal bearing is, which allows the spacer to be removed for adjustments.
Use anti seize paste on the nut.

To fix a worn out wheel hub you can add shims to hold the bearing in place.
Do this only for the spare wheel and not for the active wheels.
Do not put one large shim to one side. It will off-center the bearing.

If possible, try to slide three smaller shims at each 120° position.