In light of Ash's recent IVA fail on the steering column, i'll be needing to review my own
Now just thinking a little outside the box .... How's about turning some bosses to join the extension tube/bar, say 60-80mm long !!??
This would allow the use of 15mm soild bar or perhaps 19mm OD thick wall tube.
Once welded, they could be drilled & fitted with a roll pin as a fail safe ( Way OTT, I know )
I know on my own build, a thinner extension shaft would give me a tad more clearance of the offending chassis rail
Like I say .... Just thinking aloud