In reply to kingjam:
Try tapping the wall were you wish to attach the ply baseboard to find the joists, make sure you drill into these - across several different joists if possible to spread the load. Your wall is not likely to be MDF, but plasterboard and may not take the direct outward/downward load if drilled/glued in directly.
Also check - if the doorway is back into the main house then it may have a solid brick or concrete surround behind the plasterboard. I would suggest you use something big like radiator Rawls. I used (10mm by 1100mm) these to fit my beastmaker, it sits on top of a doorframe, goes through the plasterboard then there is a 3-4cm gap before it goes into a solid lintel. Any potential downward flex is killed by the hefty doorframe which I reinforced with some no more nails.
Good luck!