same framebuilder that Custom builds the rack would braze in a threaded boss into your fork crown, then you get both painted or powdercoated.

there are long discussions on how the handling is better with such a front load by having a fork with more offset, and so less trail ... thats the way the original Parisian newspaper bundle haulers had their bikes designed. BQ rhapsodizes on that stuff

Then again, off the shelf. There are Racks that would mount using the same bolts that hold the V or cantilever brakes on. Old Man Mountain offers a few

and there is a Tubus Smarti , seems in European distribution , if you are over there , EZ, Retailer shipped across the pond if not . pannier racks.

and there is Paul's Flatbed rack , an aluminum/Wood slat rack somewhat of the type you pictured,
But uses 1/2" aluminum rod stock
for the support rail around the cargo area and the support struts, into a CNC socket fitting in the edges..

And there is a bracket that fits around the steerer tube above the headset or the stem Quill .

Quite adjustable , no welding involved.

http://www.paulcomp.com/flatbed.html