Here are some resources that you might find useful. My original blog post about this project: http://cosmodro.me/blog/2011/mar/25/rhombus-square-iskew...
Step 2: Make a shim to read track 1
Track 1 of a magnetic card is .11inches closer to the edge of the card than track 2. Since the Square reader is set up to read track 2, if we stick something in the reader to raise the card by .11 inches, the read head will be aligned with track 1 rather than track 2.
You can create a shim by cutting a .11 inch strip from another card. I have also found that the twist ties from cheap garbage bags are just about right too.