Depending on how thick the plastic is (and this method isn't suitable for something more than 1-1.5mm thick), a No. 23 blade on No. 4 scalpel handle (load blades with pliers, not your fingers!). Fetch a true-steel-guided edgeT-section ruler (to keep your fingers away from you-know-what). It works, but it's mightily scary... Probably best, though, to practice a few times on something before committing to the important stuff!