Tip 1: don't use metal tube for the steering housing

My first steering housing was made of gas-tube with a wall thickness of 3 mm. This proved to be insufficient. Especially in the beginning of your career as a penny farthing rider, you tend to use too much force on the handlebars in an attempt to keep your balance.
The steering column is a part on the bike where a lot of forces are concentrated: the weight of the rider is distributed on the steering column through the steering pin ; the force exercised on the handlebars is also transferred to the steering column and the tensioner bolt to keep the steering pin nice and stable between the two extremes of the steering column also puts the steering column under a lot of stress.
This was the result of a few rides with the original gas tube steering column:

As you can see, the steering column opened up by the forces exercised on the bike. You can see the paint that chipped off in the corners of the opening for the steering pin.

The new steering column was made of a solid stock metal bar and the wall thickness was kept at 5 mm.
This proved to be the right strength and everything has been perfect since then.