So you got 296 for both wheels and you need 292 for the rear.
What nipples are you using? Any chance of dropping one notch in nipple length to compensate for the long spokes?
How 'bout going to 4X on the rear? Depending on flange height and spoke number that'll eat up some extra mm. You might find another match by both going up in no of crosses AND in nipple length.
Spokes CAN be cut and rethreaded, "all" you have to do is; find a bicycle store that has the required machine, isn't too busy to tackle the job, and is nice enough not to charge you an arm and a leg to do it...