@axl -
I put a square of thin vellum paper under the mat to raise the center a little.
Brilliant - and very simple. That's a simple and ingenious solution, I would not have though of. NICE !

And yep - stick with pla.
There are only a couple of very specific reasons for ever using abs. And you will most likely find that easier to use filaments like pet or pet-g will do a better job anyway :-)