If the ply is inside and the balsa outside, try to slit at the seam between them with a #11 blade, go slow and make the slot longer than you need by a couple mm, enough to get a small piece of razor saw into it to "clean "it out. You can trim the hinge, and in some cases, just cut it in half. This isn't my idea as much as SIG's, this is how they show to mount the drop-down door and lift-up window on their Cub kit. Instead of gluing it in, if you get a fit that is tight enough, you might want to pin the hinges, will allow you to remove the door if you ever need to without worry of tearing up the doorframe. DuBro makes some really small (thin) hinges designed for 1/2A and .20 size planes that would work well for door hinges on a plane the size of yours, after all they don't have to take aerodynamic loads.