After a lot of experimenting with various types of ‘whaletail’ style guards i bought one of these last weekend :-
http://www.decathlon.co.uk/mtb-rear-mud-guard-id_5993871.html
and bodged it to fit the swingarm on my Marin Wolf Ridge.
The two downward-facing arms fit snugly into the swingarm, i fitted some vibration-damping sticky-backed foam from work and drilled three holes in each arm. Used some cable ties to fasten the guard to the swingarm and voila! One close fitting guard that is tightly fitted (no wobble) to the swingarm, has approx an inch clearance between tyre and guard, and doesn’t prevent me getting right off the back of the saddle.
All for £7.99 and a few cable ties.