We've recently launched a Fork Mount Roof Rack, with options for 15mm or 20mm axles. Our adapter is a piece of machined aluminium that replicates your hub so that you use the axle that comes with your fork. We believed this to be the strongest safest option possible.
Last time I checked Pace/DT Swiss were the only MTB forks that specifically prohibit the use of this type of rack because of the Carbon Fibre. Certainly Rock Shox and Fox are fine with fork mount roof racks.