While we are on the subject of making windows this is a good opportunity to demonstrate how I cut clear styrene.
Clear styrene is rather more brittle than white styrene so you can save yourself a lot of grief by not trying to cut around corners or curves, chances are the sheet will shatter and always in the wrong way.
Plan your work and cut only straight cuts; this may seem wasteful but much less so than failed angled or curved cuts.
First I measured the width of the strip I required; in this case 17 MM.
I scratched a small marker where I wanted the cut to be:
Employing my utility knife and a fairly accurate combination square I scored a fine line on the surface of the clear styrene. I would suggest a hard surface and not a foam cutting mat as shown here but the cutting mat shows the scored line better.
Once you have the scored line, you can either remove the square or chance deepening the score with repeated passes with the knife but bear in mind that each pass increases the possibility of slipping or cracking the styrene.
I removed the square after the second pass and then after relied on the depth of the score to keep the knife from slipping.
I made half a dozen more passes until the score was deep enough to attempt snapping the line.
Try to snap the line gently and evenly apply pressure along the length of the cut instead of a localized point.
If it seems you need undue pressure score the line deeper rather than applying more pressure which could shatter the sheet.
The result is a cleanly cut strip.