I use the acrylic from Lowes as well. It is a perfect solution for me. I also make a sleeve of the lining fabric to slip it in, seems to stay in place better. And when that bag has lived out its days, I save that piece of acrylic for the next one.
I know that Lowes will cut it for you, but did you know that you can cut it yourself? Just mark your lines, score really well with a box cutter, and snap apart. I use blue painters tape to wrap the edges and corners so it doesn't cut through the sleeve or bag.