How are you rewarding the find? Is the value of reward high enough?
Personally, I'd break this behavior down through elimination. Make the find really easy. The instant Frieda even comes close to indicating (not sure if you've trained an indication yet) I'd mark it, reward, and move away from the boxes. I would simply not give her the opportunity for exploration.
Gradually, if you're using a high enough value reward, her drive to get to the find should increase. And when you see this increase, I'd start adding back in those distracting boxes with other dog scents on them. And I'd add them slowly.
Also- you might take a look at Susan Garrett's "It's Yer Choice (IYC)" which is a strategy to get dogs to maintain focus on their work. IYC generally starts through food training, but once the dog has the concept, there is no reason that you can't apply IYC to really any distraction.
C-ATCH Pimg - DOB: 2/06, RATO, UJJ (x2), DJA, HIT, CA, CGC, High in Games, Reserve High in Standard- CPE Nationals 2014
RATCHX High Jinks vom Neuanfang - DOB 9/12 (Gotchya Day: 1/23/2013) CA, agility superstar in training