Here's a quick tip if you already own a pistol type soldering gun. You can replace the wire tip with a small piece of wire shaped however you like to create a small hot wire knife for cutting foam. I made a variety of shapes to try out different things and you'll find that the smaller the wire the better it works. In fact, the largest wire shown in the picture would barely get past warm and didn't cut at all. The best were the small sqare wire and the small sort of round wire. They heat up very quickly and cut very fast. Using a smaller wire to make multiple cuts will definitely work faster for hogging out a hole but a larger wire such as the large rectangular wire work well for cutting precise large shapes such as a battery box. You can use these shapes for scooping out holes for batteries, servos, receivers etc. or they work very well for getting a straight cut making windows or cutting out slots for ailerons etc.
Hope you enjoy this tip. As always please comment. Let me know why you do or don't like what I'm doing so I can make these better. Thanks for reading.
Remember, if at first you don't succeed, sky diving is probably not for you.