Pesky 1/4" Seams

I posted earlier about how none of my 7" squares came out square on the purple & green quilt I'm making. The troubled started here: 

The nice little white line on my bobbin cover would suggest a quarter inch seam. However if you line your fabric up so that you can still see the line you end up with an 1/8" seam, not 1/4".  If you line up the fabric just over the line you lose sight of it and end up going over (3/8") or sewing a really crooked line. Well as least I do. 

I decided that on the next quilt I would make absolutely certain that I'd get nice 1/4" seams.
So I ordered this:

 An Ultimate Seam Guide from here. I loved that it came with two little plastic guides that I could run my fabric right up against. Sadly it was just too big for my machine. The platform needs to be at least 5 inches in order for the guides to work and sadly mine is a piddly inch before it starts to slope down. If my machine didn't slope it'd work just fine.

So I came up with some cardstock & tape:

So far so good. The ultimate test will be when I starting putting together the blocks. Fingers crossed.