I haven't used Blender before I'm a 3ds max user myself, but if its similar there should be an inset command in polygon mode that will allow you to inset a window shape.
Try looking for an polygon inset tutorial for it on youtube.
Out of curiosity what compressor are you going with. I was about to order the TC90T AIRBRUSH COMPRESSOR WITH VEDA 130 AIRBRUSH but the website states they will only ship with in the UK. I've sent them an email regarding this and I'm waiting an update.