If you have a windshield glass shop nearby have them cut a pexi glass square the unfinished size of your block. The shop here cut me a 18 1/2" square for $9.

That is exactly what I use! I think it works just fine.

Doesn't matter - You can still use it to square up - measure with your ruller and use Post Its to mark the lines on the mat. I suspect there will be some that are pretty dang close to how you want to square.

sew in metric

I'd take tape or a marker and mark out the desired size on your gridded side. You can still use the existing lines to help keep edges straight.

Here is an easy way to square up blocks using freezer paper. http://quilting.about.com/od/quiltin...wap_blocks.htm

a local hardware store or home depot store should be able to supply you with plexiglas or other clear plastic. this can be cut into squares of different inch increments. with a fine-line permanent marker, draw diagonal lines and cross lines at the center of each side. that will give you the center of each block. mark each one permanently with the size unfinished: 5-1/2. 8-1/2, etc. that will provide the seam allowance. you can square up any size block very frugally.