I've used PC-7 to do repairs on the exterior cladding with very good results. I haven't used it on any of the interior panels that I can remember but it'd probably be the best match & plenty strong. You can order it online or pick it up at True Value.
It's 2 part epoxy that mixes into a pretty thick paste. Press it into the crack as best you can with a putty knife. You can put a piece of tape across the 'face side' & work from the back to save yourself some sanding once the epoxy has cured.