Tranum: I bought my mat from JoAnns a number of years ago. Too bad that yours is developing a small 'hump' but I like the idea of using a weight to hold it down. I have one mat that has a wave on one edge and I keep forgetting to put it on my driveway in the summer to see if the sun would melt it enough to get it flat and those weights would be perfect to use to hold it flat.
grann of 6: I was lazy and didn't use pins to hold the fabrics together and, boy oh boy, did I have to work to get it square by reserging parts of it twice.
Congrats on using 2 of your never-used rulers.