34.9mm front derailleur on steel frame? Can I fit a 34.9mm front Shimano derailleur on my steel frame? I understand that steel frames are narrower. However, is there some sort of adapter that I can use, or can I even wrap the frame around there with some inner tubes to "fatten" it up? Will this be sufficiently stable?

Any thoughts will be appreciated, thanks.
You need a derailleur shim

The one linked adapts a 34.9 mm derailleur clamp to a 31.8 mm seat tube.  I haven't seen other sizes but I'm sure it could be augmented/replaced with something homemade.