Any foam-core modeler will tell you that it’s important to make a corner that looks flawless so you can easily paint it and finish it out. A good way to do that is to use a rabbet cut — a notch cut along the length of the edge in question — to mate the two pieces. The bad news? Cutting rabbets in foamboard is a pain — unless you’ve got a foamboard rabbet cutter like this one.
Here’s how it works: first, you run the cutter along the edge you want to notch, placing the foam core in one of the channels. Then you flip the cutter over and use the other channel for a second cut to clear the notch from the board. If you’ve ever used a hand-held matte cutter, this is very similar.
The $10 investment won’t kill your budget, but the time saved on trying to align corners without tedious frustration or flimsy joints might spare you a few gray hairs. The only drawback is the unit is really only setup for 3/16″ foam core, so don’t plan on stocking up on off sizes.