6-

How exactly was the part not straight? Was there

1. a gap when you pressed the pieces together?
2. One of the pieces bowed up or down from the board when laid flat?
3. The laser cut line was not square to the sheet?

If your problem is either #1 or #2 it could just be due to different densities/grain patterns in the balsa sheets, or uneven swelling from humidity. Just lay the parts flat, weight then down and use a good gap filling glue in the joint. Be careful not to overflow the joint or you will leave a ridge that will never sand off properly. You can even leave the joint ever so slightly low and fill in later with light filler.

I would be very surprised at case #3, I never saw that in my SE. In fact I used to work for a company that makes laser cutters and that kind of problem is practically impossible unless you stick your hand in the machine and hold the board at an angle (something that cannot be done without disabling the safety cutoff switch on most equipment). But thats not to say impossible.

If the joint is really mismatched, you could have gotten a bad cut. Just call up Sig and I'm sure they would give you a replacement.