As you get more efficient, fatigue will be less of an issue.
If you can't rest your elbow easily, I'd rec just resting the hand holding the condensing lens against the forehead rest (you could even hook your pinkie on it).
I think you mean direct ophthalmoscopy. Anyway, there are elbow rests you can buy (or make), but it's really just something extra to fumble around with. As optsuker said, you should be able to get away without it. The lens boxes are metal with a padded cover, so they should work if you must have one. I'd try to get used to working without one, though.
Could you mean indirect ophthalmoscopy via a slit lamp? This is probably one of the hardest technique to perform for more than a couple of minutes if your arm is short.
I have used the lens case to prop my arm up, but the case flattened once and I now use a folded up towel or even a thick paperback book.
Oops, that's what I meant. I have actually used a direct ophthalmoscope with a condensing lens at the bedside for AC evals, but slit lamp ophthalmoscopy is definitely indirect, not direct. Sorry, if this caused any confusion.