Yeah, don't laugh. I've done it...
Years ago, I worked at a sign shop in Mississippi that will remain unnamed... We sent a guy out to measure some Burger King faces. He got the height right, as well as the corner radius, but the width was off, by 11''. He quit several weeks later, and moved on. When the faces came in (and we are talking about pan face, embossed, painted faces), and we went out to install them, and they didn't fit, the boss was livid.... she thought that we would end up closing the shop because of the foul up. Her brother came up with the idea of taking the sign down, take it back to the shop and narrow the sign to fit the faces. We did, took it back to the shop (less than a mile), and started cutting it down. The only problem is it was a center pole, so we had to take 5 1/2" from each side. And we cut it with a sawsall, and welded it back up with the stick welder on the back of our crane truck generator in the back yard... I ended up working 17 hours that day, and working through the night. I got the worst flash burn in my eyes... We got it finished that night, and went and re-installed it the next day...
Back to the OP. For a piece of flat polycarb that small, buy a new piece... Do the job correctly, and don't skimp.