If there are not enough threads to let the headset screw all the way down, Know Sheldon
shows a hybrid threadless headset , threaded up top and held in place with those threaded nuts and locknuts.


actually 2" , 5 cm is not that much, a long Quill stem like Nitto Technomic will span the longer distance. to reach below the headset.

You have the headset adjustment secured with a through threaded nut, (?)
I did a spacer of steel Tube, and then put the top headset Nut on top of the steerer tube
and secured the styeel tube spacer with the Top Locknut ..
So through that portion it is double wall steel.

its on my self contained loaded touring bike , already used it on a 6+ month tour
of Ireland and Scotland. with no issues ,

It even functions as its own headset press , when I put the CK headset together.