Ben--
You may need to use a smaller "seam allowance" or, depending on your fabric, you may need to take triangular notches out of the 1/2 inch "seam allowance" to allow it to lie flate against the base of the mouthplate.
If you're using contact cement, pull your fabric over the "lip" and onto the base of the mouthplate. Smooth out as many of the "puckers" as possible and make the remaining "puckers" as small as possible. Next, using scissors, trim the "puckers" flush with the mouthplate. You will see that the pieces you remove are small triangles. The last step is to flatten the two cut edges, making the fabric appear as seamless as possible.
Hope this helps.
--melissa