Need a custom "road boost front derailleur mount"

I am currently in the process of building a custom bike, which has boost spacing in the rear and uses a boost mountain 2x crankset, with 46-30 chainrings. The bike will have drop bars and a GRX 2x11 groupset.

What I wanted to know is if anyone can make a front derailleur mount that can move the GRX front mech (47mm chainline) out to the boost chainline (51mm). I believe a special clamp or mount is used by Salsa to accomplish this on the Cutthroat.

Can someone here make something like this? Or point me to someone who can?

What diameter seat tube are you using?

No experience with this, but in theory it could work:

34.9 clamp minus 28.6 seat tube => up to ~5mm of adjustment

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The eccentric shim Daniel recommended should work nicely. It’s the Jtek Eccentric Front Mech Shim if you need to source it outside of the us.

Another option could be to use the Salsa Custom FD Mount they made for the cutthroat. Could be cool to see that piece on a metal frame! Since it’s made for a carbon frame, it’s entirely possible it won’t work with a round tube but it’s cheap enough to purchase and try out for fun.

If you’re looking for good crank and bb options, there has been discussion about this topic on the 29-er Gravel Bike with Boost Spacing - What Drivetrain?! thread.
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You could get one 3D printed with the extra spacing or file up a 4mm thick spacer to match the base of the braze on FD mount and braze it all together. If you are permanently fixing it.

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Most people here could make a custom front derailleur clamp. If you’re building the frame you could likely make yourself a custom front derailleur clamp. Here’s one for inspiration: