Here's an alternative: I used a length of 3/4" alloy tube *inside* the existing aerobar, to support sections of a cut-down old aerobar.

It means abandoning any warranty on the monobar, as you have to drill a small hole to hold the new extensions in place. But it works very well for me, and gains about 7cm in reach and the same in hand height.

It does mean extra load on the extension mounting bolts (though no more than if using the plug-in + long extensions), and it's not clear how well the extensions will cope with the extra forces at the shifter end. But for me, this shifts the load onto my elbows and my grip on the extensions is very relaxed, so I personally think the trade-off is worth it.