Or break from a 'break box' on your own side of the headstring as your own pocket. John, I've also found this to be a great way to entice non-One Pocket players to play a little One Pocket
If they won't try this, then I like to suggest
9-ball bank
How would you rack 5 or 9? If you racked like regular 9-ball, the wing ball would be a very high percentage make I would think -- which might mess the balance up. For three balls, I've tried three lined up on the spot, rather than a triangle -- the three ball line-up being a fairly common situation in any standard One Pocket game. The problem with 5 balls lined up is that the 5th one cuts right into your pocket!
Six balls would make a standard triangle, which might work -- all you would have to do is both go to four and whoever makes the first legal ball, spots it up after they finish their inning, and then they only need 3 more, while their opponent still needs four. That would make the ball count come out right.
Ten balls could work the same way (going to six balls each and the first player to score spots one), but then you're getting up pretty close to needing eight anyway, so why bother.
I've never tried 6 or 10 balls or anything else other than 1, 3 or 15 balls so if anyone else has, I'd be curious to hear how it worked...