Actually, the stuff is quite stable. That is why many photograph and picture frame makers use it to back expensive photos or paintings. The technique for cutting into it is to determine the depth, take you razor sharp cutting tool, wrap tape many times to leave out only the amount of blade that represents the depth you want. You then go around the perimeter of the cut vertically, going straight in plunging till you see the surface reach the tape edge. you do this, then gently, cut around the perimeter and you will isolate the part to be removed, if you do it right, you will also have a panel that can be hinged to open and close. you either pull off the center paper off the foam, if you need to save it, if not you the do the same thing you did around the perimeter inside on the interior, cutting it till there are a bunch of little squares, at the center you go side ways, using a nice machinists rule works best, and the little squares pop of and this process take far less time than it did for me to write this post. You could have down a 4" x 2" panel in the that time, and I type fast.
All this is done with finesse, and extremely sharp blades. This is also why I recommend starting to use more 110 lb. card stock, as you won't get that boxy look and can star making wings that have an airfoil shape and look like the crafts you are trying to design. With 110 lb. card stock, you could cut a strip, use a dowel the same thickness as the foam board, and make leading a trailing edges that would enhance the models you have already made greatly. I could get a piece of foam board and show you, but I think you can do this on your own. Better too make one exceptional model than many O.K. ones. Only one life.
With the model you made below, you could use the same technique to make the engine inlets tapered to the actual inlet, and add much realism to the model. You have the foundation for fantastic models, you just need to take it up a notch. If your satisfied with what you have made, that's fine too, but the reality remains the same. You can see this by looking at the works of all the other members. People join this forum and virtually every single one's has seen there modeling technique grow because of interaction with other members, and are producing, and designing their own models that could be sold commercially, and many have.
