Thought I'd mention this in case others are wondering if it's worth trying.
Folks have come up with a few ways to attach FDs to thicker-than-the-market-is-motivated-to-serve downtubes. Starting from a two-piece commercial solution of "braze-on" FDs* and adaptor clamps**, tomtomtom123
figured out the flaws in market adaptors and had a non-flawed one 3d-printed in alumnum. Falconista went old-school, and just
forced the FD and the adaptor to mate using a hand file. Meanwhile, starting from a band FD with a too-small band, Celika
cold-formed it to the needed radius using a vise and a cylinder.
Being more of a Dremel guy, I just succeeded by the method stated in the thread title. It wasn't totally straightforward. A gusset on the 2009 Eco3 prevented FD from being placed at height where chain would clear cage crossbars in all gears; to deal with that I had to amputate part of the FD, specifically the flange which houses the limit screws. I also needed to trim the FD cage a little, because at the viable height it interfered with the large chainring. But now I have a 28/48 double, and a 16.5" low gear, and can ride up the scenic route to my place - with its 15% grade - to the amazement of neighbours.
* relic term, confusing for two reason: the things actually brazed on were never the FDs, but frame features they screwed on to; and those features aren't really brazed on any more; "screw-on FDs" would nail it
** screw-on bands (from Litepro; haven't heard of others) with alleged equivalents of those frame features, that sometimes don't quite work