custom build box for 12' sub with kenwood amp..thanks people for looking.

 

I was hoping someone could help me figure out what I am doing wrong. I am trying to build a box for a jdm car. I have heard many other people say they had success using the tape method, but I just can't get it right. I have tried using the "1~1.25" method, but I feel like what I get out is at least an inch too big, and when I start trying to trim the wood it doesn't look like the corners are matching up. I am using a tape measure and a paint ruler to try to get the seams to match up when I am done, but I feel like a professional would have a better tool than what I am using. I feel like it must be user error, but I cannot figure out what I am doing wrong.