While the long bolt is an easy solution it is ome I would try and avoid. there will be alot of strain on the bolt when the mudguards rattle over bumps etc, this could lead to premature failure of the bolt, the mud guard stays are easy to bend and fashion a bracket for the rack.
I must have been thinking about this in my sleep because I woke up with the same thoughts! I am worried less about the strain caused by the mudguards, more by hanging a rack on the end of the long bolt. Also - I tried spreading the fixings on the rack to the required separation and it put the rack under more stress than I would feel happy with under load. (Those bits are aluminium and needed forcing outwards by a total of about an inch.)
I'll take my time and come up with a solution, probably based on finding a suitable piece of metal in my junk box or cellar. People laugh at the amount of old bits and pieces that I store, but it is amazing how often I can find suitable brackets, bolts etc. for this kind of job.