[MrForbes]

If there is one bumper on each side, then each side needs to be a minimum of 8". If you want to have a gap between bumpers on a single side, that is, have two bumpers on that side, then that side would need to be more than 16" to fit the two bumpers, plus the gap (I assume the only reason you want two bumpers with a gap between them, is so you can have some mechanism poke out of it)