I used 32NB pipe as it was off the shelf, was just smaller than the cross section of the 50x150 I used for the steps and you can buy bends for it. Comes from 10NB and up, small and long radius bends plus tees/crosses etc.

I don't have any cutouts but here's the weld
It's on a D2, here's pre extensions.
Yes there's a gap because of the D2's curved doors, no I don't care. It's somewhere to put your toes.