[villagetinker]

Jimbomaybe, At this point I am confused by your posts, but here is what I would do:
I would leave the 'space' (second post) and just place the plastic panels on the sleepers.
I would use the tapcon screws you mentioned however yo may need to be careful that you do not crack the plastic.
I would not seal the top or bottom seams of the plastic panels to the floor or ceiling, this would allow some breathing in case there is moisture build up I THINK.
All of this is assuming that you are talking about solid plastic panels. If you are just after a covering, you might look into a perforated panel (like the soffit panels), this would provide covering and allow ventilation for moisture.
You might also check with a home inspector like Frank DeAngelo on the matter to see if something is being missed, and if there are other concerns not being addressed, like recessed outlets, etc.
Good luck with your project.