The solution? Extend your rappel device away from the belay loop. I generally use a full-length (48-inch) sewn sling doubled into two redundant loops. To set this up, tie an overhand knot in the middle of the sling, forming the two loops. Girth-hitch one end of the sling into the two connection points of your harness or the belay loop; clip the other to the belay loop with a locking carabiner. Then clip the other end of both loops to the locker on the rappel device. This method adds about a foot of space between the back-up and rappel device.
This technique has many advantages. The extra space under the rappel device allows you to use both hands to brake the rope. With the pivot point higher relative to your body, it’s easier to stay upright when rappelling with a heavy pack. Also, the locking carabiner that connects one loop of the extension sling to your belay loop can serve as a convenient anchor clip-in point during multiple rappels. Finally, you have the option of attaching the prusik to your belay loop instead of a leg loop, improving the security of the system should the main rappel fail.
Disadvantages include the possibility of the rappel rope rubbing against and burning the extension sling (use a nylon sling, which is more resistant to melting than Spectra or Dyneema); plus, with the rappel device closer to your head, the risk of hair getting caught is higher.
Eli Helmuth’s guiding company is based in Estes Park, Colorado (guide.climbinglife.com)