I have a steady betty, but before I get into that, I'm wondering about your quarter inch seam. How are you measuring it?
If you sew your two patches, pull the fabric out of the machine, and measure just the seam allowance without opening the fabric or pressing the seam, you might still have an inaccurate seam allowance.
Sew 2 patches that are exactly 2" wide. After you press the seam and open out the patches, measure across the fabric, NOT the seam allowance. They should measure 3.5" across. If they don't, then you know your seam allowance is inaccurate and you can fix it.
Now on to the steady betty. They are expensive and I do have one. I've found that covering my ironing board very snugly in cotton duck works just as well as the steady betty, and is quite inexpensive. The cotton duck grabs the block and prevents it from slipping.
You might check your pressing technique, also. I have a friend who discovered that when she pressed from the back side, often times she was pressing a pleat or some extra fabric on the front. Make sure your fabrics are fully opened when you press.
HTH!