The yoke spacing on the BOB Trailer is quite a bit wider then a typical front fork mount bike rack, you could probably modify one with wider spacers and use a trailer skewer (or similar fabricated part) to mount the trailer yoke. The wheel tray will need to be long enough for the span of the trailer and the fork mount will need to be high enough (or the tray low enough) otherwise the trailer will stall on the wheel tray when you try to mount the trailer yoke in the fork mount of the bike rack.
I had an extra roof top, frame clamping, bike rack on hand so I designed and fabricated a bracket that combines the yoke mounting of the trailer with the roof top mounting of the bike rack so it mounts to anything in the same way that the roof top bike racks mount:
![Trailer on Roof Rack](