cutting

Although my issue is with a tank part, this should be applicable to all kinds of modelling, hence why I post in the general area. I'm building an E-100 from Amusing Hobby right now and have come to a halt with the muzzle break. It's just not good enough since it's solid moulded with fake slots...

When cutting a simple shape like a square or a rectangle, I mark out the lines on the sheet, put a (usually steel) ruler on and cut with a sharp craft knife. The problem is that the ruler (no matter what it is made of) slips and the cut wanders, ruining the cut. I have tried using double-sided...