Haven't read all of the posts, but I put mine in the sun/hot area with either clamps or heavy books on the bubbled up area and it flattens out nicely.

One time I put rectangle cake pans or really any type of flat container would do and poured boiling water in the pans,I repeated this a couple times till the mat was flat. then placed heavy books on it for a couple of days, that has been a couple years ago and it is still good as new.

I will try this...gosh, but these mats are costly, anything is good to try in order to same cash. It seems that winter and loss of good strong sunlight has arrived.