I used a 10x12 PVC 20mil liner for my bog filter outside. Considering that its mostly full of gravel, then filled with plants, and has wood trim covering the top and edges, there's very little that's exposed to sunlight.
We do get very cold winters here, down into single digits every winter, and below freezing for several weeks to months at a time. The liner has been out there for over a year now, and is still just as good as new.
I think for indoor use, you could use one of the pvc ones and be just fine. Or, you could double up two of the 20mil pvc liners, and it would still probably be cheaper than a 45mil epdm liner.