This may be old news for some, but I think it works pretty good. I have found that UHMW (Ultra High Molecular Weight) plastic works extremely well for a cutting surface for PE parts. It is rigid and flat enough to not let the PE part distort while cutting, and is soft enough to allow the knife to pass into it without the dulling effect of glass, aluminum, or lexan. If you are in a quandry as to where to find the stuff, check McMaster Carr. It's not expensive. I do recommend a piece that's at least 1/2" thick. Also, you can find it at any machine shop or job shop that may be local. Drops are usually a buck or 2 and perhaps free if you ask nicely.
Also, you can drill a blind hole in an unoccupied corner and use this for CA glue storage to prevent drying out. Nothing sticks to the stuff. And it's extremely slick and wear resistant. It is basically a bearing material.
Hope this helps someone.