Can a dremel cut plastic? Yes, but that's a huge oversimplification...
Depending how much you need to cut off the options would be a big cutting disk (bigger means it's easier to notice it's not perfectly in line with the cut you want and harder to go off line once you've started cutting, but be warned big disks are more fragile. I usually use around 2" or 5cm carbon ones but that was for metal...) or going round the whole thing with a grinding bit which will require a lot of concentration, patience and some good headphones/earplugs. Lose concentration and press too hard in one spot and you've got a dent in the lip when you're done.
Either way be prepared for lots of dust and/or shards of plastic flying everywhere
)
The other problem is it's plastic, and plastic melts. If you have a enough unwanted material at the top and are committed to doing this you should experiment and find the right speed setting for the job.
Once the shortening is complete you'll want a big sheet of fine sandpaper (plus coarse and medium if you used the grinder option) and a glass table to smooth the cut edge and maybe round the outer edges slightly, and maybe a lick of flame from a mini torch or blue flamed lighter to shine it up - again, test what happens on a bit you cut off as a test, and do it outside as there may be fumes.
Good luck!
